Gemspec with bundler

I have simplified the `Gemfile` and `gemspec` files and dropped rails 3.x 2.x references there.
And since bundler 1.2.0 will have `bundle config --local`, I don't think we need the environment variables in `Gemfile` for local gem paths.

In the future we could use [appraisal](https://github.com/thoughtbot/appraisal) to test against multiple gem versions.

@rsim what do you think?