Replace gem dependencies for listen with a dependency on listen.

Listen 1.0.0 and later uses these gems as dependencies itself so we don't need to specify them here.  In order to ensure that version 1.0.0 or later is used, we should require that gem.
Remove deprecated version parameter for the guard-rspec plugin.
Fix guard behavior on OS X when using built-in Ruby.

The built-in version of Ruby does not use deadline.  This causes issues with the Pry interactor in guard, preventing it from seeing changes automatically.  The rb-readline gem is a pure Ruby version of readline and can be used as a workaround.
Remove guard-cucumber workarounds.

As of version 1.2.1, guard-cucumber properly handles the feature_paths setting.  We therefore don't need to monkeypatch Guard::Cucumber.
Use guard-shell to handle starting and provisioning Vagrant VMs.

Since we use halt_on_fail for the group, we no longer need the callbacks in the Cucumber guard.  If the RSpec or Cucumber guard fail, the guard that handles Vagrant won't run.
Remove clone of Guard::Cucumber.

Guard::Cucumber was originally cloned because the callbacks appeared to be duplicated when multiple Cucumber guards were run.  Since we're no longer using callbacks, we can safely undo this.
Change names of groups to be more succinct.

The name change also makes the purpose of each group more explicit.