Bundle exec jekyll in .travis.yml

Prevent `command jekyll not found` errors by wrapping the Jekyll command into Bundler's context during CI.

This fixes the pre-merge testing of pull requests, and does not affect the rendered output. Also bump Jekyll to the latest version for consistency sake.

## Before

```
$ jekyll build
/home/travis/build.sh: line 165: jekyll: command not found
The command "jekyll build" exited with 127.
```

## After

![screen shot 2013-08-05 at 2 26 37 pm](https://f.cloud.github.com/assets/282759/912204/a3f5ee34-fdfc-11e2-8345-1c46881286c2.png)

https://travis-ci.org/project-open-data/project-open-data.github.io/builds/9870682

Fixes #99.
/cc @seanherron