Use .ruby-version and allow for overrides

Implements the canonical solution, suggested in
[this](https://stackoverflow.com/questions/16087120/should-i-check-in-ruby-gemset-and-or-ruby-version/16738020#16738020) Stack Overflow answer by the maintainer of RVM.

- Add specific ruby version as a dotfile
- Allow any ruby gemset name
- Allow override of .ruby-version and .ruby-gemset using .rvmrc
Moves .ruby-version and .ruby-gemset to the root.

They need to be placed in the same directory as the command
```sh
make server
```
Would be run from.

See discussion
[here](https://github.com/google/WebFundamentals/pull/694).

Closes #693
Closes #694
Merge pull request #724 from jbnicolai/issue/693

Use .ruby-version and allow for overrides.