Fatal exception under JRuby: uninitialized constant ActiveSupport

The dependency chain for 3.5.0 under JRuby combined with how active support was being required and used here was causing some issues and completely breaks shoulda under JRuby 1.7.x.

Simply requiring should-context on the JRuby platform results in a fata exception.

Steps to reproduce:
```ruby
rvm use jruby
gem install shoulda
ruby -e "require 'shoulda'"
```

Result:
```
NameError: uninitialized constant ActiveSupport
  const_missing at org/jruby/RubyModule.java:2631
         (root) at /Users/brandon/.rvm/gems/jruby-1.7.4@sandbox/gems/shoulda-context-1.1.2/lib/shoulda/context.rb:7
        require at org/jruby/RubyKernel.java:1054
        require at /Users/brandon/.rvm/rubies/jruby-1.7.4/lib/ruby/shared/rubygems/custom_require.rb:36
         (root) at /Users/brandon/.rvm/gems/jruby-1.7.4@sandbox/gems/shoulda-3.5.0/lib/shoulda.rb:3
        require at org/jruby/RubyKernel.java:1054
        require at /Users/brandon/.rvm/rubies/jruby-1.7.4/lib/ruby/shared/rubygems/custom_require.rb:60
         (root) at -e:1
```

This pull request resolves the issue and all tests pass successfully under:
* MRI 1.9.3
* MRI 2.0.0
* JRuby 1.7.4