Latest Version of Vyper (0.1.0b6) Breaks Populus

* Populus Version: 2.2.0
* OS: linux

When using Vyper with version 0.1.0b6, the compilation process fails with this error:

```
bytecode = '0x' + compiler.compile_code(code).hex()
AttributeError: 'dict' object has no attribute 'hex'
```

This is the fix that I used for now:
```python
- bytecode = '0x' + compiler.compile(code).hex()
- bytecode_runtime = '0x' + compiler.compile(code, bytecode_runtime=True).hex()
+ bytecode = compiler.compile_code(code)['bytecode']
+ bytecode_runtime = compiler.compile_code(code, ['bytecode_runtime'])['bytecode_runtime']
```