[MINOR][BUILD] Ignore ensime cache

Using ENSIME, I often have `.ensime_cache` polluting my source tree. This PR simply adds the cache directory to `.gitignore`

Author: Jakob Odersky <jodersky@gmail.com>

Closes #9708 from jodersky/master.

.gitignore

@@ -50,6 +50,7 @@ spark-tests.log
 streaming-tests.log
 dependency-reduced-pom.xml
 .ensime
+.ensime_cache/
 .ensime_lucene
 checkpoint
 derby.log