(Maint) Move to newer beaker

rbvmomi released a 1.8.0 that broke ruby 1.8 compatibility. We tried
pinning to an older beaker, but that wouldn't have worked because of the
way beaker depends on rbvmomi. Beaker 1.3.1 pins itself to an rbvmomi
that we should be able to use and so using 1.3.1 should solve the issue.