Include Travis CI badge in the README.md.

Separate from the automated Travis CI e-mails, it would be handy to see
a visual cue.  Plus, I would like to tickle the Travis CI trigger to
test HEAD against a recent runtime outside of my workstation, for I
think something is misconfigure locally for me.
Merge pull request #1 from matttproud/master

Include Travis CI badge in the README.md.