Add Ubuntu support to params.pp

Previous to this commit, if the module was used on an Ubuntu system, the
path to the unicorn executables were not entered into the
/etc/defaults/unicorn_<app> file causing the init script to not be able
to launch the app.  This commit adds the path to the Ubuntu executables
to the params.pp file
Merge pull request #20 from ccaum/ubuntu_support

Add Ubuntu support to params.pp