Fix brakeman issues

 * Installed brakeman gem[1], ran tool, found one issue, fixed it, and created this pull request.
  * The issue was the use of "^" and "$" is github_url validation string (vs. "\A" and "\Z").
 * Brakeman output after the fix:
  * Scanned/Reported, then Total
  * Controllers, 11
  * Models, 9
  * Templates, 32
  * Errors, 0
  * Security Warnings, 0 (0)

[1] For more info: http://brakemanscanner.org/