Fix puppetlabs_spec_helper warning on Ruby 1.8

Ruby 1.8 doesn't have the #source_location method. So instead parse the
filename out of the string representation from Proc.
Release 1.2.3
Merge pull request #18 from gds-operations/psh_warning_ruby_18

[Fixes #13] Fix puppetlabs_spec_helper warning on Ruby 1.8