Document how to change RPC port

* Populus Version: 1.5.0

### What was wrong?

Documentation here discussed about how to configure Web3 http://populus.readthedocs.io/en/v1.5.0/config.html

However it doesn't provide anything useful for the user

* The documentation examples do not look like the actual `populus.json` file created by `populus init` (see below)

* The documentation does not tell *what parameters* are available or *how* to give different parameters to Web3 provider

#### Cute Animal Picture

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