Remove the upper cap on ply version
Remove the upper cap on ply version

Latest released version of ply is 3.8 now. Since it is very unlikely
that subsequent releases will break anything in yaql, it's preferred
to remove the upper cap at all.

Change-Id: I805d37917b65771736b54babe453edda2bba6e31

stackforge/yaql,openstack/yaql