Remove the explicit ruby requirement in Gemfile.

Shall we try letting go of Ruby 1.9? Ruby 2 works with Rails 3.2, for
some time now, so it seems stable:
https://github.com/rails/rails/pull/9406

Motivation: It might be better to avoid forcing beginners -- coming to
this beginner-friendly project -- to use the older Ruby version, from
the get-go.

In LocalSupport specifically, after removing this line from the
Gemfile, the unit tests still all pass and clicking around a bit, the
site renders well blackbox-wise too.