(maint) Make PuppetSpec::Language emit more helpful stack traces

This change was required to be able to see anything relevant regarding
where a problem originated. The fix is to patch the stacktrace
for failed expectencies. While not ideal there is very little that can
be done with Rspec 2 to fix this except reimplementing the support for
produces and fails to be matchers. (Rspec 3 has composable matchers that
may make this simpler).

A ticket has been logged about further improvements to the two support
functions (produces, and fails).