[SPARK-4890] Ignore downloaded EC2 libs

PR #3737 changed `spark-ec2` to automatically download boto from PyPI. This PR tell git to ignore those downloaded library files.

Author: Nicholas Chammas <nicholas.chammas@gmail.com>

Closes #3770 from nchammas/ignore-ec2-lib and squashes the following commits:

5c440d3 [Nicholas Chammas] gitignore downloaded EC2 libs

.gitignore

@@ -55,6 +55,7 @@ dev/create-release/*final
 spark-*-bin-*.tgz
 unit-tests.log
 /lib/
+ec2/lib/
 rat-results.txt
 scalastyle.txt
 scalastyle-output.xml