PROTON-406: Fix installing the Ruby bindings.

Previously it checked for RUBY_VENDORLIB_DIR as provided by Cmake. But
this variable is not provided by older versions of CMake. Additionally,
older versions of Ruby did not provide a vendorlibdir. In those cases,
the behavior is to now get the Ruby sitearch dir instead.

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1515795 13f79535-47bb-0310-9956-ffa450edef68