Testbeds with rails 4

I am authoring [a gem](http://github.com/sinisterchipmunk/jax) that depends on jasmine-rails. I plan to support all of Rails 3.1, 3.2 and 4.0, so I have an interest in seeing #33 get implemented with backward-compatibility to Rails 3.x.

> Starting to realize I may need dummy apps at multiple point releases to prevent myself from going insane

So, this is exactly the sort of thing I have been doing in my own gem. It's a non-trivial process, so I've pulled out the supporting code into a new gem called [testbeds](http://github.com/sinisterchipmunk/testbeds). Then I forked jasmine-rails and created a [testbeds branch](https://github.com/sinisterchipmunk/jasmine-rails/tree/testbeds) which depends on it.

Then I merged #33 into the new testbed stuff, and sure enough Rails 3.x was broken :(. So I fixed Rails 3.x in the [testbeds with rails 4 branch](https://github.com/sinisterchipmunk/jasmine-rails/tree/testbeds-with-rails-4), which is where this pull request comes from.

If this pull request is for whatever reason not acceptable, you could take a look at the [testbeds branch](https://github.com/sinisterchipmunk/jasmine-rails/tree/testbeds) to make use of just the testbed support, without the Rails 4 changes to `lib/` that were merged in from #33.

One more note about this pull request: I reverted commit b9d69856a17fdbdff7f1886b07858cd4295fc44c because it caused specs not to run under phantom; and after reverting it all tests passed under all testbeds.

The travis builds are passing for this pull request under Rails 3.1, Rails 3.2, and Rails 4.0-rc1:
https://travis-ci.org/sinisterchipmunk/jasmine-rails/builds/7085835

The travis builds are passing in the [testbeds branch](https://github.com/sinisterchipmunk/jasmine-rails/tree/testbeds) branch under Rails 3.1 and Rails 3.2 but obviously they are failing under Rails 4.0-rc1 because 4.0 support is not included on that branch: https://travis-ci.org/sinisterchipmunk/jasmine-rails/builds/7085947 .