Set up Travis CI

Broke up the monolithic shell script into separate steps so the output
is clearer when it fails. These scripts are just thrown into a gist for
now at https://gist.github.com/jasonmp85/9963879. I had to enhance them
to allow a `PG_PRELOAD` environment variable to inform Travis what libs
should be preloaded (i.e. if the extension being tested has hooks).
Show build status for master branch only
Add missing link reference in README

We want the badge to link to Travis.
Merge pull request #1 from citusdata/enable_travis

Set up Travis CI