Cleanup CI

[jcrist]

• Use pre-built docker image uploaded to docker-hub
• Cleanup docker file to give smaller image
• Cleanup environment scripts
• Don't sleep arbitrary amount of time, wait for hdfs to be setup properly using an indicator flag

All together these changes drop 5-6 minutes off our travis ci time per build.

[martindurant]

OK, cool. I was also at some point using dockerhub mdurant/hadoop , don't know if there are significant differences.

[jcrist]

There's not much of a difference in the environment, but there's a few more options in the startup, and the way it interacts with travis is more efficient.

[martindurant]

Passes, so good enough for me.