Use environment variables for travis, remove redundant test gemfiles

This commit updates the travis config to remove the heavily redundant test gemfiles and instead add an optional environment variable `$RAILS` which enforces the version of actionmailer requested for the tests.

This variable is then set by Travis-CI instead of one of the Gemfiles.

This should remove the need to manually update the Gemfiles with new dependencies each time and thus increases DRY :)