migrate from buildout to virtualenv

also drop Python 2.6 compatibility

Change-Id: I179d28d31ab3126352dbd1f8c829ff80c166052d
Reviewed-on: http://review.couchbase.org/31053
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>