Use new rspec syntax and couple more improvement

Hi Andrew,

First of thanks for build such a great TDD/BDD gem for Vim development.
The reason for the pull request is I really like to use newer version of rspec (3.x instead of 2.x).

Could you review this pull request and see if you can merge it to the branch?

If you want me to add/remove anything please let me know.

### Detail of the pull request:

- Use the most recent version of the gems
- Use new expect syntax for rspec
- Use require_relative instead of require when possible
- Add pry to rake task for quick prototype/testing.
- Misc code improvement

This may be useful to the whole community.

Thanks,
Burin