Upgrade to use rspec 3 and improve code structure

Hi Andrew,

Once again thanks for writing a great gem.

Here are the quick summary of the PR:

####  1st commit -  Update gemspec file

- Add rspec 3.1.0 to the gem file 
- Add debugger related gems (pry, pry-byebug)
- Update .gitignore file to include .ruby-version 

#### 2nd commit - Update the spec files to use 3.1 expect syntax
- Update rspec to 3.1.0
- Add debugger related gems
- Add .ruby-version to .gitignore

### 3rd - commit - improve gem structure (3rd commit)

- Use require instead of require_relative when possible
- Move some logic in `lib/vimrunner.r` to `lib/vimrunner/vimrunner.rb`

Please ignore/delete my other PR (eg https://github.com/AndrewRadev/vimrunner/pull/37) and use this one instead as it contain smaller commits that you can easily review before merge.

If you do need me to make any adjustment, please let me know.

@mudge  I have included some of your idea to this one as well.

Best,
Burin