Accept Python executable path from environment

REPROZIP_PYTHON and REPROUNZIP_PYTHON control which Python executable to
test with.
Adds back Python 2.6
Uses python2.7 to run reprozip on 2.6 tests
Merge branch 'removes-sh-tests' into 'reprounzip-py26'

Conflicts:
	.travis/test.sh
Provides check_output() for Python 2.6
Installs reprozip in 2.7 venv
Don't run 'reprounzip graph' on 2.6