Add JEKYLL_ENV limitation for git remote

[at-ishikawa]

Why

In order to find out the logic to configure NWO using git remote depends on JEKYLL_ENV easily.

What

I got the errors when I ran JEKYLL_ENV=production bundle exec jekyll serve for GitHub Pages.
The error is something like this.

  Liquid Exception: No repo name found. Specify using PAGES_REPO_NWO environment variables, 'repository' in your configuration, or set up an 'origin' git remote pointing to your github.com repository. in /_layouts/single.html
             ERROR: YOUR SITE COULD NOT BE BUILT:
                    ------------------------------------
                    No repo name found. Specify using PAGES_REPO_NWO environment variables, 'repository' in your configuration, or set up an 'origin' git remote pointing to your github.com repository.

When I checked this code, I figured out that git remote is only checked if JEKYLL_ENV is either of development or test from the below code.
https://github.com/jekyll/github-metadata/blob/master/lib/jekyll-github-metadata/repository_finder.rb#L63

By the way, I'm a beginner for ruby, jekyll, and github pages, so please let me know something is wrong.

Related issues

• The comment from issue 91

[parkr]

Thanks!

[parkr]

@jekyllbot: merge +docs