Output the dependency versions for this build of Pages.

This allows easy access to the Gemfile bundler directives, which may
be easily used to tweak versions for upgrades, etc.

An alternative to #71.

## Usage:

### Individual dependencies

```bash
$ bundle exec github-pages versions
```

### `github-pages` gem git directive

```bash
$ bundle exec github-pages br [branch_name]
```

### Using in your project

```bash
$ bundle exec github-pages versions | pbcopy
# comment out the gem "github-pages" directive, replace with contents of clipboard
# bump versions of gems you're interested in.
$ bundle update # does it work?
$ bundle exec rake test # does your app/site still build with the new versions?
$ # if it all works out, go submit a PR to pages-gem.
```