(PDOC-9) Add full system test

In order to help ensure that the strings module is not broken
during development, add a full system integration test which can
run at regular intervals to ensure that the module is working as
expected.

The test installs the string module as well as a test module which
contains the different types of puppet and ruby code that the strings
module can document. It then runs strings and ensures that the HTML
generated is correct.

Additionally, make some changes to the Gemfile to allow strings to
be run as a gem even if it isn't installed in the modulepath. Also
make a few changes to the unit tests to make them compatible with
the version of rspec required by Beaker.