Fixes #611 Get Hadoop version from EMR

Instead of mapping hadoop versions to AMI versions we now create a job flow without specifying the hadoop version. The hadoop version is set by querying the job flow with DescribeJobFlow
Code review changes
Merge pull request #615 from sudarshang/611_get_hadoop_version_emr

Fixes #611 Get Hadoop version from EMR
Merge branch 'master' of github.com:Yelp/mrjob into simplify_bootstrap