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.
The default repo that everyone inherits in any Maven build is Sonatype's repo, which has just been called "Maven Central" for as long as I can remember: http://search.maven.org/  It's not an Apache repo.

The reason I changed the name is that its ID in the default Maven parent pom is "central". Right now, it's not actually overriding the default. Maven Central repo is included twice in the list of repos, which does very little harm except to cost a duplicate check to Maven Central when an artifact isn't found.

Still, it seemed more reasonable to actually override it as intended. I suppose that otherwise, you'd be leaking your (failed) requests for artifacts even after this change to secure these requests, although that's very minor.

Anyway that's why I changed it to "central", since that's its ID in the default Maven parent.