Disables check for Python bug 13676 in reprounzip

This only affects us if we read from the trace database. Reprounzip now
only uses the YAML config file, so this is no longer an issue, except
for the 'reprounzip graph' command.
Adds check for Python bug 13676 in unpackers.graph
Moves smoketests from test.sh to functional.py
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
Merge branch 'reprounzip-py26' into 'master'
Updates CHANGELOG