bundled cacert.pem with --ssl-ca-file PATH option (#480)

* Add a bundled cacert.pem from curl's website
* Using the --ssl-ca-file PATH option overrides ENV var SSL_CA_PATH
* Having none of those allow you to use the default, the bundled cacert.pem
* Octokit's SSL config uses this new setting
* Rake task to update_ssl_ca_file
* Rubocop target 2.2

lib/github_changelog_generator/options.rb

@@ -43,6 +43,7 @@ module GitHubChangelogGenerator
       :release_url,
       :simple_list,
       :since_tag,
+      :ssl_ca_file,
       :token,
       :unreleased,
       :unreleased_label,