If the user has python2.7 installed but it's not the default interpreter
on their system, they might get confused when they invoke the parser and
get a message about needing to install 2.7 even though they already have
it installed. Add some instructional messages about this situation to
the 2.6 invocation warning message as well as the README.

Change-Id: Ib1ee7df39199f715b236dbda2f75093ba0501648