SPARK-1084 (part 2). Fix dependency-related build warnings

This is the follow up to https://github.com/apache/incubator-spark/pull/637

This contains dependency-related changes. It removes the "exclude *" dependencies that cause Maven warnings. The changes below are sensible to the best of my knowledge, but I would definitely like @rxin to look at this as he made some of these exclusions, and there may be a subtle reason for them that I have overlooked.

* `org.sonatype.sisu.inject` seemingly does not appear in the dependency graph of any transitive dependencies anymore. It can therefore be removed if so.
* `org.xerial.snappy` is used directly by Spark, in core. It is unnecessary to remove it from the transitive dependency graph; these overridden dependencies had no effect anyway.
* `org.codehaus.jackson` is not used in Spark (anymore?). `com.fasterxml.jackson` is -- Jackson 2.0 -- but this is not directly relevant. The direct dependency on it in Streaming can be removed, and then there seems no reason to exclude other versions from transitive dependencies.