Ensure same 1.8.7 version is used when cross-compile

Use the same version of Ruby 1.8.7 across the board to avoid RVM
telling us the version is not installed.

I'm actually not sure why rvm will say that `1.8.7-head` is not
installed, compared when using 1.9.3 or 2.0.0 it always install
the latest available patchlevel.

:confused: