(PUP-2828) Pin to 1.9.3 branch of vendored ruby on Windows

Previously, acceptance tests on Windows would install the default branch
of our vendored ruby repo. This is a bit fragile.

This commit makes it explicit that we are using the 1.9.3 branch,
which contains the older set of windows gems.

Newer development to support x64 will be done in the 1.9.3-x86 branch.