SPARK-1254. Consolidate, order, and harmonize repository declarations in Maven/SBT builds

This suggestion addresses a few minor suboptimalities with how repositories are handled.

1) Use HTTPS consistently to access repos, instead of HTTP

2) Consolidate repository declarations in the parent POM file, in the case of the Maven build, so that their ordering can be controlled to put the fully optional Cloudera repo at the end, after required repos. (This was prompted by the untimely failure of the Cloudera repo this week, which made the Spark build fail. #2 would have prevented that.)

3) Update SBT build to match Maven build in this regard

4) Update SBT build to not refer to Sonatype snapshot repos. This wasn't in Maven, and a build generally would not refer to external snapshots, but I'm not 100% sure on this one.