[SPARK-4225][SQL] Resorts to SparkContext.version to inspect Spark version

This PR resorts to `SparkContext.version` rather than META-INF/MANIFEST.MF in the assembly jar to inspect Spark version. Currently, when built with Maven, the MANIFEST.MF file in the assembly jar is incorrectly replaced by Guava 15.0 MANIFEST.MF, probably because of the assembly/shading tricks.

Another related PR is #3103, which tries to fix the MANIFEST issue.