Drop Python 2.6 Support

[jmchilton]

As announced on -dev.

[peterjc]

Can we add something like this to setup.py as well please?

if sys.version_info < (2, 7):
    sys.stderr.write("ERROR: planemo requires at least Python Version 2.7\n")
    sys.exit(1)

[nsoranzo]

Python 3.4 is missing here.

[jmchilton]

@peterjc Done, thanks for the suggestion.

@nsoranzo Let's wait until the tests consistently pass before adding that. It is definitely the goal though.