Update dev tools

Update tools and plugins - Mostly to help with development and contributions.

### Bugfixes
- REALLY fix #222 (superclass mismatch error - see lib/guard/rspec.rb for notes)

### Major changes
- Check out the new Guardfile template

Even if I screwed up a rule or two - I think it's worth it, so check it out.
For more ideas see: https://github.com/guard/guard/issues/649

### Improvements:
- use RSpec 3.1 for development + fix specs
- use guard-rspec, guard-rubocop, etc.
- use Rubocop from master (by a fixed ref)
- add Rubocop configs
- fix Rubocop offenses

### Controversial

I set the gemspec dependency to 2.99.0, because there's simply no reason to use
2.14:
- `transpec` is too awesome to not consider upgrading
- RSpec >= 3.0 is too awesome to not consider upgrading to it
- 2.99.0 = 2.14 with lots of feature and deprecations for a smooth upgrade
- 3.0 is no longer "new"

Any feedback welcome.

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