Fix to sample code for href in ReactPropTypes

Error should be thrown in the previous condition is "not" meet. And the href propType is described as optional in the comment, but the null check was missing.
Wrapped line to 80 chars in ReactPropTypes sample
fix link in community roundup #18
Merge pull request #1767 from jgebhardt/fixlink

fix link in community roundup #18
Cache node_modules on Travis

This isn't actually enabled yet for public projects, but this will help
us speed up builds when it does get enabled.

http://docs.travis-ci.com/user/caching/
Merge pull request #1750 from protron/patch-1

Fix to sample code for href in ReactPropTypes