Exclude vendor folder by default

Travis CI recently changed the way it vendors gems (with the `--development` flag), which causes the default build to fail due to conflicts with Jekyll core living in `./vendor/`, and thus being processed by Jekyll. Have to imagine putting Gems in `vendor` is a pretty standard practice, and is only going to become more common place as Jekyll popularity and maturity continues to evolve. ([example](https://travis-ci.org/github/choosealicense.com/builds/10119798))

![screen shot 2013-08-13 at 8 59 42 am](https://f.cloud.github.com/assets/282759/954519/5bef82d6-0418-11e3-80eb-dadb1d2a37a6.png)

In the vein of sane defaults, and the implied promise that things should Work Out of the Box :tm:, let's just add `vendor` to the exclude list. Alternatively, or in addition, giving that templematic post a real date, or prefixing some folders with `_` may also work, although be less ideal.