Add .ruby-version to 1.9.3

We don't actually execute any Ruby in this project, except for during
the build process.

I think this will alleviate some issues when installing gems:

    sudo: no tty present and no askpass program specified
    Sorry, try again.

    Gem::Exception: Cannot load gem at [/var/lib/gems/1.9.1/cache/sass-3.2.12.gem]
Use bundle exec when building Sass in production

So that we use the copy of the sass binary in vendor/cache.
Merge pull request #112 from alphagov/add-ruby-version

Add .ruby-version to 1.9.3