scala.io.Source now implements java.io.Closeable

allows users to write a single function for dealing with Input/Output
streams and Sources. It also allows Source to be used in
"try-with-resources" statements in Java > 7.

No tests since this is a binary and source compatible change.
Merge pull request #3837 from hamnis/source-closeable

SI-8686: scala.io.Source now implements java.io.Closeable