Bumping rspec dependency

[myoung34]

Allows the use of rspec-its which depends on rspec >= 3.0.0

[myoung34]

Had to bump .travis.yml ruby version as well because travis breaks on

An error occurred while installing listen (3.1.5), and Bundler cannot continue.
Make sure that `gem install listen -v '3.1.5'` succeeds before bundling.
Gem::InstallError: listen requires Ruby version >= 2.2.3, ~> 2.2.
Using vagrant-spec 0.0.1 from source at .
An error occurred while installing listen (3.1.5), and Bundler cannot continue.
Make sure that `gem install listen -v '3.1.5'` succeeds before bundling.

Also, the registry of shared examples is no longer usable in RSpec 3 (http://rspec.info/documentation/3.4/rspec-core/RSpec/Core/SharedExampleGroup.html#registry-class_method). That information is now private and not accessible: https://github.com/rspec/rspec-core/blob/master/lib/rspec/core/shared_example_group.rb#L204

[hashicorp-cla]

All committers have signed the CLA.