PROTON-570: If Ruby dependencies are missing, then Ruby is not built

Previously, the entire build would fail if the dependencies were
missing. Now, instead, it will just not build the bindings or run the
tests if any are missing.

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1590755 13f79535-47bb-0310-9956-ffa450edef68