fix($animate): remove the need to add display:block!important for ngShow/ngHide

Since ngShow/ngHide animations add and remove the .ng-hide class, having to remember
to write display:block on your own is a hassle and leads to problematic animation
code. This fix places a default on the animation for you instead.

Closes #3813
docs(ngShow): calirfy info about the .ng-hide class and the display style
docs(misc core): Use `@kind function` instead of `@function`.
chore(travis): run npm install twice

This is not optimal solution at all, but I can’t think of any better.