Usage statistics: adjust JS scheme to current scheme

When the CMS is served over HTTPS, it still includes the usage statistics script located at cdn.onehippo.com over HTTP. As a result, the browser logs an error and refuses to include the script. Usage statistics will not be reported for these users.

Steps to reproduce:
1. configure HTTPS locally in Apache
2. serve GoGreen over HTTPs (e.g. on https://cms.example.com)
3. login to the CMS

Expected: no errors in the JavaScript console
Actual: the following error is logged in the JavaScript console (in Chrome):

Mixed Content: The page at 'https://cms.example.com/?1' was loaded over HTTPS, but requested an insecure script 'http://cdn.onehippo.com/s/10.0.2-SNAPSHOT/2015-6-15/cms.js'. This request has been blocked; the content must be served over HTTPS.