fix tests on Ruby 1.8.7
use https scheme for assets by default

- use https scheme for assets by default
- allow URI scheme for assets to be controlled by asset-uri-scheme
- upgrade highlightjs to 8.0, remove unnecessary JavaScript resource
- load MathJax 2.4.0 from cdnjs
- load FontAwesome 4.1.0 from cdnjs
- add tests for asset-uri-scheme