Define Coveralls excluded folders in rails_helper.

- Exclude the .bundle directory from Coveralls

When using the `bin/wad` script, the gems are saved in a `.bundle`
directory, which Coveralls thinks should be checked for test coverage.

Here, we exclude it using the `add_filter` method that SimpleCov
provides.
Merge pull request #534 from codeforamerica/exclude-wad-from-coveralls

Define Coveralls excluded folders in rails_helper.