Specify Rails version in Gemfile to avoid bundler hell

Fixes this problem that I was having:

    $ bundle
    Fetching gem metadata from http://rubygems.org/.........
    Fetching gem metadata from http://rubygems.org/..
    Bundler could not find compatible versions for gem "rack":
      In Gemfile:
        jquery-rails (>= 0) ruby depends on
          rack (~> 1.2.1) ruby
    
        capybara (>= 0) ruby depends on
          rack (1.5.0)
    
    Bundler could not find compatible versions for gem "bundler":
      In Gemfile:
        jquery-rails (>= 0) ruby depends on
          bundler (~> 1.0.0) ruby
    
      Current Bundler version:
        bundler (1.2.3)
    
    This Gemfile requires a different version of Bundler.
    Perhaps you need to update Bundler by running `gem install bundler`?
    Bundler could not find compatible versions for gem "rails":
      In Gemfile:
        librato-rails (>= 0) ruby depends on
          rails (>= 3.0) ruby
    
        jquery-rails (>= 0) ruby depends on
          rails (3.0.0.rc2)

Although I don't know what side effects it might have on users of older versions of Rails. I believe it shouldn't affect the gem user because I changed a development and not runtime dependency.