Fix Travis build failing

To fix the following offense

```sh
Offenses:
Gemfile:11:5: C: Style/StringLiterals: Prefer single-quoted strings when you don't need string interpolation or special symbols. (https://github.com/rubocop-hq/ruby-style-guide#consistent-string-literals)
gem "jekyll-github-metadata"
    ^^^^^^^^^^^^^^^^^^^^^^^^
14 files inspected, 1 offense detected
```

Added comments

Gemfile

@@ -8,7 +8,8 @@ versions = JSON.parse(open('https://pages.github.com/versions.json').read)
 
 gem 'github-pages', versions['github-pages']
 
-gem "jekyll-github-metadata"
+# For 'Improve this page' links
+gem 'jekyll-github-metadata'
 
 group :development do
   gem 'colored'

_config.yml

@@ -40,7 +40,7 @@ plugins:
   - jekyll-redirect-from
   - jekyll-seo-tag
   - jekyll-coffeescript
-  - jekyll-github-metadata
+  - jekyll-github-metadata # For 'Improve this page' links
 
 
 sass:

_includes/sidebar.html

@@ -1,5 +1,6 @@
 <div class="sidebar">
-
+  
+  <!-- Adds the `Github Link` for the corresponding page page -->
   {% github_edit_link "Help improve this page" %}
 
   <a href="#" data-clipboard-target="#license-text" data-proofer-ignore="true" class="js-clipboard-button button">Copy license text to clipboard</a>