Add ability to specify a symlink for r10k.yaml

This patch allows me to have a symlink for r10k.yaml that points to
where the config file is located. This is useful if you want to put the
config file in a non-standard location while not having to tell r10k to
look in a different place for the config file.
Add support for Puppet 3.5.1 in Travis
Add parameter to optionally include prerun_command class

This patch allows you to parameterize the inclusion of
r10k::prerun_command and keep you from having to include a separate
class. The code optionally does an include on the class in order to
maintain backward compatibility. Recommend that in a future release the
code from the prerun_command class is moved into the r10k class where it
is currently being included.
Merge pull request #41 from ghoneycutt/add_symlink_for_config_file

Add ability to specify a symlink for r10k.yaml