Must have at least one provider backend configured

### What am I doing wrong?
When I do

```
populus init
populus compile
py.test
populus deploy --chain tester Greeter --no-wait-for-sync
```
it's all good, but then with
```
populus deploy Greeter --chain CRS --no-wait-for-sync
```

I get a

```
> Found 1 contract source files
- contracts/Greeter.sol
> Compiled 1 contracts
- contracts/Greeter.sol:Greeter

Traceback (most recent call last):
File "folder/env/epe-py3/bin/populus", line 11, in
sys.exit(main())
File ".../python3.5/site-packages/click/core.py", line 722, in __call__
return self.main(*args, **kwargs)
File ".../python3.5/site-packages/click/core.py", line 697, in main
rv = self.invoke(ctx)
File ".../python3.5/site-packages/click/core.py", line 1066, in invoke
return _process_result(sub_ctx.command.invoke(sub_ctx))
File ".../python3.5/site-packages/click/core.py", line 895, in invoke
return ctx.invoke(self.callback, **ctx.params)
File ".../python3.5/site-packages/click/core.py", line 535, in invoke
return callback(*args, **kwargs)
File ".../python3.5/site-packages/click/decorators.py", line 17, in new_func
return f(get_current_context(), *args, **kwargs)
File ".../python3.5/site-packages/populus/cli/deploy_cmd.py", line 43, in deploy_cmd
deploy(project, logger, chain_name, wait_for_sync, contracts_to_deploy)
File ".../python3.5/site-packages/populus/api/deploy.py", line 78, in deploy
provider = chain.provider
File ".../python3.5/site-packages/populus/chain/base.py", line 137, in provider
"configured\n{0}".format(self.contract_backend_configs)
ValueError: Must have at least one provider backend configured
OrderedDict()
```

This is my project.json
```
{
"version": "7",
"compilation": {
"contracts_source_dirs": [
"./contracts"
],
"import_remappings": []
},
"chains": {
"CRS": {
"eth": {
"default_account": "0x0000000000000000000000000000000000000001"
},
"chain": {
"class": "populus.chain.ExternalChain"
},
"web3": {
"provider": {
"class": "web3.providers.rpc.HTTPProvider",
"settings": {
"endpoint_uri": "https://127.0.0.1:8545"
}
}
}
}
}
}
```

I have my own parity instance running, on port 8545, on a private chain.

What haven't I understood?

Thanks a lot!

#### system info
* Populus Version: x.x.x

```
populus version
Error: No such command "version".

populus --version
Error: no such option: --version
```
(I have the version installed today, via pip)

* OS: osx/linux/win

```
uname -a
Linux ... 4.9.0-4-amd64 #1 SMP Debian 4.9.51-1 (2017-09-28) x86_64 GNU/Linux
```

#### Cute Animal Picture

> put a cute animal picture here.