Add .ruby-version file

If I ran all tests with "bundle exec rake", I encountered the "no script in
input"/ruby -S issue. This solved that for me. It also seems useful:
 1) it documents the project in a semantic way
 2) it lets tools like rbenv and rvm work automatically
Merge pull request #86 from alphagov/add_ruby_version_file

Add .ruby-version file
Upgrade Ruby version in travis settings

To match .ruby-version just added.