Changed test runner instructions to run under 'bundle exec' to match CI....

... The tests were failing under the prior instructions.

This is small potatoes, but a few tests were failing for me with the old instructions but they were passing in TravisCI. Checking I see that Travis runs under 'bundle exec' and that does indeed pass at the command line. I could delve in to see why 'rake spec' is failing, if that is preferable.