This fixes the tests for the case where `ruby` is not the same
Ruby interpreter as the one you are running the tests against. For
example, the `ruby` in yout $PATH might be Ruby 1.8, but you are running
the tests under Ruby 1.9 (or vice-versa).
If ohai was installed by Rubygems, it doesn't need to call `require
'rubygems'` because the bin stub created by Rubygems will do that
Also, only add the local lib/ directory to the $LOAD_PATH if ohai is
actually being run from a source checkout.