COMPAT/BUG: Ignore numexpr < 2.0
in both computation.expressions and computationr.engine. Issues with API
compatibility.

Fixes #5857, #5855, #5845.

@jtratner can you fixup....?
 @jtratner ?
 doing it now - thanks for the second ping.
 I'll merge this once I get it to pass - still a few kinks to work out
 u prob need 2 change a build maybe 2.6 
 wow - never seen one minor change break so many tests!
 something weird going on that requires going deeper, going to circle back tomorrow
 @jtratner ?
 I'm working on it. There's a few weird test failures that I'm not grokking
right now. (sorry I have way less free time than I used to)
 closing.. @cpcloud putting up a new PR
 okay thanks @cpcloud 
 np!
