Update Rakefile to make rspec optional

Currently rspec is a hard dependency to using the Rakefile for puppet. This
means that users on platforms without native rspec packages can't use the
Rakefile to do other things, like build packages. This commit moves the require
on rspec and rspec/core/rake_task into a begin/rescue block and wraps the call
to Rspec::Core::RakeTask in an if defined? conditional so it is only invoked
if Rspec is loaded and available.