[SPARK-2627] space out test report sections

Everything looks cramped and it’s hard to tell at a glance where
sections begin. Adding a blank line between sections should fix that.
[SPARK-2627] download pep8 at runtime

See the discussion here:
https://github.com/apache/spark/pull/1744#issuecomment-50982162

Get the pep8 utility at runtime so that it’s not required to be
installed on the build server.