Added proper version handling and tests

The install url has to exist outside of params, because the version number can be overridden. 
(causes puppet to run wget https://dl.bintray.com/mitchellh/serf/0.2.1_linux_i386.zip -c -O /tmp/serf_0.3.0.zip, when version => 0.3.0 is specified)

This moves that logic into the install class where the $::serf::version param is really set, and adds tests for it. 

This also bumps the default version to 0.3.0, which is the current latest.