Pin the Ruby version for development

It doesn't affect any users of the gem itself, but it makes working on the gem
more predictable.
Remove unneeded dependencies in Gemfile

They're already in the gemspec.
Fix unresolvable development dependency tree

The number of combinations of dependencies was so large that Bundler was taking
a very long time (forever?) to resolve the dependencies. Pinning the dev
dependencies doesn't affect users of the gem.

Also require Rails 3.2 and above - not strictly required in this change, but
all of our apps are on 3.2 and above anyway.