Extra keys in `$ref` values within configuration should throw an error.

* Populus Version: 1.6.x
* OS: all

### What was wrong?

Any *reference* in a configuration file which has *extra* keys should throw an error at the time the configuration file is loaded and validated.

Example: https://gist.github.com/poiresel/0cf1aba442be17fead55c00fb2d3c546#file-populus-json-L110-L115

#### Cute Animal Picture


Start with the configuration loading and validation entry point, using the linked gist as the reproduction case for a reference containing extra keys. Trace how that configuration is checked and add coverage showing that loading such a file reports an error; done means the invalid reference is rejected during validation.

Written by the indexing model from the issue text.