Add Gemnasium badge to readme

Gemnasium syncs with your GitHub repositories. It parses your projects'
dependencies and notifies you when new versions are released. This
commit adds the badge to the readme to help keep us in check.

Change-Id: Ifb2eeea8e87464ad773685a8d7a62898e3505564
Reviewed-on: https://gerrit.causes.com/33803
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>
Bump nokogiri dependency 1.6.0 -> 1.6.1

According to Gemnasium[1], there were two CVE's that were fixed in this
version of nokogiri.

[1]: https://gemnasium.com/gems/nokogiri

Change-Id: Ib1c16840e97fc1342b52ecaaa7e415a6f6109e6b
Reviewed-on: https://gerrit.causes.com/33804
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>