Increase alternatives priority

Increase alternatives priority so 2.0 is preferred over 1.8. This brings it into line with the packaging for [1.9](https://github.com/brightbox/deb-ruby1.9.1/blob/master/debian/ruby1.9.1.postinst) and [2.1](https://github.com/brightbox/deb-ruby2.1/blob/master/debian/ruby2.1.postinst).