Insert rspec setup code into generated binstub

- solves https://github.com/jonleighton/spring-commands-rspec/issues/18

This code relies on code in [this pull request](https://github.com/rails/spring/pull/329) and should NOT be merged until the other pull request is merged. At that time, the gemspec may also be updated to depend on the newer version of the spring gem.
This caused an error for me unless preceded with `require rspec/core`. Please see https://github.com/jonleighton/spring-commands-rspec/issues/18#issuecomment-50848941