Pull the Hadoop version from the environment.

This introduces two environment variables that you can use to control
the version of Hadoop that you build against. The Spark project
uses `SPARK_HADOOP_VERSION`, which is supported. By analogy, we
also support `SHARK_HADOOP_VERSION`, which has precedence over
`SPARK_HADOOP_VERSION`. If neither of these environment variables are
set at build time, the defaults from the build script are used.

The algorithm looks like this:

   1. Use `SHARK_HADOOP_VERSION` if set.
   2. Use `SPARK_HADOOP_VERSION` if set.
   3. Use the default that is hard-coded into the build definition.

Conflicts:
	project/SharkBuild.scala