Build lib in prepublish hook
Build lib in prepublish hook

...and check for lib first in postinstall hook, same as Radium.

(Aside: if we wanted to change to something easier on the eyes than the Node `stat`
snippet, I came up with `cd lib/.. || npm run build-lib` which should be
nearly as cross-platform.)

Fixes FormidableLabs/victory-bar#12

/cc @ryan-roemer @boygirl

FormidableLabs/victory-animation,FormidableLabs/victory-core,FormidableLabs/victory-core,FormidableLabs/victory-animation