Hydrate division operands when it makes sense

This PR addresses some issue with number comparisons, specifically cases involving divisions. Currently `1/2` becomes a delayed node, because of this it's evaluated in cases like `1/2 == 0.5` resulting in a `false` comparison.

Fixes https://github.com/sass/libsass/issues/590. Specs added sass/sass-spec#137, https://github.com/sass/sass-spec/pull/200.