Add health check

This pull request adds the recently open sourced [health check gem](https://github.com/github/pages-health-check), which checks GitHub Pages sites for common DNS misconfiguration issues.

It also moves the bin to Mercenary, and adds tests, to be a bit more robust.

**Edit**: Also a human-readable version output:

```
➜  pages-gem git:(health-check) github-pages versions
+-----------------------+---------+
| Gem                   | Version |
+-----------------------+---------+
| jekyll                | 2.4.0   |
| jekyll-coffeescript   | 1.0.0   |
| jekyll-sass-converter | 1.2.0   |
| kramdown              | 1.3.1   |
| maruku                | 0.7.0   |
| rdiscount             | 2.1.7   |
| redcarpet             | 3.1.2   |
| RedCloth              | 4.2.9   |
| liquid                | 2.6.1   |
| pygments.rb           | 0.6.0   |
| jemoji                | 0.3.0   |
| jekyll-mentions       | 0.1.3   |
| jekyll-redirect-from  | 0.6.2   |
| jekyll-sitemap        | 0.6.0   |
+-----------------------+---------+
```

/cc @github/pages, @parkr
I'm generally :cry: about adding more dependencies. Any way to avoid this or otherwise lighten our load? Maybe optional dependencies.