spec: updates for rspec-puppet 2.x and rspec 3.x

spec: updates for rspec-puppet 2.x and rspec 3.x

This patch aim to update our specs test in order to work with the new
rspec-puppet release 2.0.0, in the mean time, we update rspec syntax
in order to be prepared for rspec 3.x move.

In details:

  * Use shared_examples "a Puppet::Error" for puppet::error tests
  * Convert 'should' keyword to 'is_expected.to' (prepare rspec 3.x)
  * Fix spec tests for rspec-puppet 2.0.0
  * Upgrade and pin rspec-puppet from 1.0.1 to 2.0.0
  * Clean Gemfile (remove over-specificication of runtime deps of puppetlabs_spec_helper)

Change-Id: Ieaff6bc8bdd8548648ff5e76b212d4df382fa8a3
Card: https://trello.com/c/eHXc1Ryd/4-investigate-the-necessary-change-to-be-rspec-puppet-2-0-0-compliant