feat(rvm): use .ruby-version file if it exists for default Ruby

If there is a .ruby-version file in the repository and a Ruby version
isn't specified (or it's default), it would make more sense to pick the
version in .ruby-version than whatever we have set as default.

Close travis-ci/travis-ci#1984.
rvm: only use `rvm use .` when a .ruby-version file is present

Keeping `|| rvm use default` to avoid breaking projects that doesn't
have a valid .ruby-version.
rvm: don't 'rvm use default' when a .ruby-version exists
Merge pull request #213 from travis-ci/hh-rvm-default

feat(rvm): use .ruby-version file if it exists for default Ruby