fix(ngAnimate): Unblock transitions before reflow

I have investigated this issue: http://jsfiddle.net/2fnhr/1/

From this question: http://stackoverflow.com/questions/20810791/angular-nganimate-causes-width-animations-to-jump

Commenting the "blockTransitions" call resolves the issue. But there is a comment in the source-code justifying the need of blocking transitions. So, unblocking before reflow (as suggested in this PR) also resolves the issue.

This way or another, the issue needs to be addressed.