fix NoMethodError on ShellOut calls

================================================================================
Error executing action `run` on resource 'execute[add repository]'
================================================================================

NoMethodError
-------------
undefined method `include?' for #<Mixlib::ShellOut:0x00000002c0b118>

Change-Id: Ib2f562e432373fe319eee1c8485cb112785b0f9b
Update spec_helper to include 'chefspec/berkshelf'

This minor change allows you to run rspec outside of strainer while
still having cookbook dependencies automatically handled by berkshelf
(which also means you can run individual chefspec tests).

Note that this change does marginally increase the time it takes to run
a strainer test.

A similar change was recently accepted in the openstack-compute
cookbook.

Change-Id: Ia5b5f3cc8aab853dd0e8c3b120ed178b0f0887db
Merge "fix NoMethodError on ShellOut calls"