Utilize Environment Variables to set Dependency Versions

@benbalter This should allow for easier testing. E.g.:

```bash
$ PAGES_JEKYLL_MENTIONS_VERSION=0.0.7 bundle install
# installs all gems, with jekyll-mentions bumped to 0.0.7
```

Or, just update one:

```bash
$ PAGES_JEKYLL_MENTIONS_VERSIONS=0.0.9 bundle update jekyll-mentions
# updates jekyll-mentions to 0.0.9
```