Fix over-eager caching of Now variable (#191)

Closes #185.

I gave a shot at making tests for this, but it's less trivial than it
seems at first glance. I manually confirmed this fixes the bug for now.

To explain what's going on here, you have to recognize that
`newRulesVariable` is used during constraint compilation (for constant
folding) and `newThisVariable` is used during the actual activation. We
don't want `now` to get constant-folded, so we should make sure it is
only defined at evaluation time, not before. I think this was most
likely switched on accident and probably also means that the `now`
variable was not working in custom constraints.