Deprecation warnings on test suite

* Populus Version: 2.0.0
* OS: osx

There are a few deprecation warnings running the test suite. Is this one of those "don't worry about it" type of issues or does someone need to fix this? If the latter, I am happy to volunteer myself to cleaning 'em up.


```
tests/test_greeter.py ..

============================================================================================= warnings summary =============================================================================================
tests/test_greeter.py::test_greeter
/Users/brianwheeler/eth/test-contracts/venv/lib/python3.6/site-packages/populus/chain/tester.py:13: DeprecationWarning: currentProvider is deprecated in favor of `providers`, which is now a list
self.rpc_methods = self.web3.currentProvider.rpc_methods
/Users/brianwheeler/eth/test-contracts/venv/lib/python3.6/site-packages/eth_utils/functional.py:21: DeprecationWarning: The `compose` function has been deprecated and will be removed in a subsequent release of `ethereum-utils`. It is recommended that you switch to use a library like `cytoolz` which provides similar functionality through the `cytoolz.functoolz.compose` function.
"The `compose` function has been deprecated and will be removed in a "
/Users/brianwheeler/eth/test-contracts/venv/lib/python3.6/site-packages/populus/project.py:138: DeprecationWarning: project.contracts_source_dir has been replaced by the plural, project.contracts_source_dirs which is an iterable of all source directories populus will search for contracts. Please update your code accordingly as this API will be removed in a future release
"project.contracts_source_dir has been replaced by the plural, "
/Users/brianwheeler/eth/test-contracts/venv/lib/python3.6/site-packages/secp256k1/__init__.py:228: UserWarning: implicit cast from 'char *' to a different pointer type: will be forbidden in the future (check that the types are as you expect; use an explicit ffi.cast() if they are correct)
self.ctx, res_compressed, outlen, self.public_key, compflag)
/Users/brianwheeler/eth/test-contracts/venv/lib/python3.6/site-packages/eth_abi/abi.py:91: DeprecationWarning: Automatic inference of hex encoded data has been deprecated. Please adjust your code to ensure that the data argument for `decode_single` is a byte string
"Automatic inference of hex encoded data has been deprecated. "

tests/test_greeter.py::test_custom_greeting
/Users/brianwheeler/eth/test-contracts/venv/lib/python3.6/site-packages/populus/chain/tester.py:13: DeprecationWarning: currentProvider is deprecated in favor of `providers`, which is now a list
self.rpc_methods = self.web3.currentProvider.rpc_methods
/Users/brianwheeler/eth/test-contracts/venv/lib/python3.6/site-packages/eth_utils/functional.py:21: DeprecationWarning: The `compose` function has been deprecated and will be removed in a subsequent release of `ethereum-utils`. It is recommended that you switch to use a library like `cytoolz` which provides similar functionality through the `cytoolz.functoolz.compose` function.
"The `compose` function has been deprecated and will be removed in a "
/Users/brianwheeler/eth/test-contracts/venv/lib/python3.6/site-packages/populus/project.py:138: DeprecationWarning: project.contracts_source_dir has been replaced by the plural, project.contracts_source_dirs which is an iterable of all source directories populus will search for contracts. Please update your code accordingly as this API will be removed in a future release
"project.contracts_source_dir has been replaced by the plural, "
/Users/brianwheeler/eth/test-contracts/venv/lib/python3.6/site-packages/secp256k1/__init__.py:228: UserWarning: implicit cast from 'char *' to a different pointer type: will be forbidden in the future (check that the types are as you expect; use an explicit ffi.cast() if they are correct)
self.ctx, res_compressed, outlen, self.public_key, compflag)
/Users/brianwheeler/eth/test-contracts/venv/lib/python3.6/site-packages/eth_abi/abi.py:91: DeprecationWarning: Automatic inference of hex encoded data has been deprecated. Please adjust your code to ensure that the data argument for `decode_single` is a byte string
"Automatic inference of hex encoded data has been deprecated. "

-- Docs: http://doc.pytest.org/en/latest/warnings.html
================================================================================== 2 passed, 10 warnings in 0.57 seconds ================================================================================
```

Start by running the test suite and reviewing tests/test_greeter.py alongside the warning locations shown in populus/chain/tester.py and populus/project.py. Separate warnings from Populus and its dependencies, then verify the relevant warnings are resolved or appropriately accounted for when the suite is run again.

Written by the indexing model from the issue text.