SPARK-2879 [BUILD] Use HTTPS to access Maven Central and other repos

Maven Central has just now enabled HTTPS access for everyone to Maven Central (http://central.sonatype.org/articles/2014/Aug/03/https-support-launching-now/) This is timely, as a reminder of how easily an attacker can slip malicious code into a build that's downloading artifacts over HTTP (http://blog.ontoillogical.com/blog/2014/07/28/how-to-take-over-any-java-developer/).

In the meantime, it looks like the Spring repo also now supports HTTPS, so can be used this way too.

I propose to use HTTPS to access these repos.

Author: Sean Owen <srowen@gmail.com>

Closes #1805 from srowen/SPARK-2879 and squashes the following commits:

7043a8e [Sean Owen] Use HTTPS for Maven Central libs and plugins; use id 'central' to override parent properly; use HTTPS for Spring repo
HOTFIX: Support custom Java 7 location