Allow slimit to work with ply 3.6

[moses-palmer]

Always set slimit.parser.*tab to strings

These values are expected by ply to be module names, not module instances. ply 3.6 will crash if actual modules are passed.

Please note that the actual tab modules depend on the version of ply first used. Those modules are checked in, and not modified by this commit.

[delijati]

Could this be please merged

[ale-rt]

I think this one has been superseded by #102