Add Vimrunner integration tests

This PR does not add new features neither solves bugs, but adds integration tests for indentation using [Vimrunner](https://github.com/AndrewRadev/vimrunner) gem. This exposes many bugs in the indentation provided by this plugin.

To run this tests you have to have ruby with bundler installed. Inside `vim-elixir` directory, run `bundle` and then `bundle exec rspec`