[SPARK-4890] Upgrade Boto to 2.34.0; automatically download Boto from PyPi instead of packaging it

This patch upgrades `spark-ec2`'s Boto version to 2.34.0, since this is blocking several features.  Newer versions of Boto don't work properly when they're loaded from a zipfile since they try to read a JSON file from a path relative to the Boto library sources.

Therefore, this patch also changes spark-ec2 to automatically download Boto from PyPi if it's not present in `SPARK_EC2_DIR/lib`, similar to what we do in the `sbt/sbt` script. This shouldn't ben an issue for users since they already need to have an internet connection to launch an EC2 cluster.  By performing the downloading in spark_ec2.py instead of the Bash script, this should also work for Windows users.

I've tested this with Python 2.6, too.