Travis: --use-mirrors in deprecated in pip 1.5
Settings: Specify ATOMIC_REQUESTS for DATABASES

Django 1.6 replaces TransactionMiddleware by ATOMIC_REQUESTS == True.
This allows more control over the transactions, however it also means
database backends that don't behave properly without autocommit will
break in 1.6 (such as sqlite).
initdb: Do not create a manual transaction for projects/languages/admin

This makes Django 1.6 complain as sqlite will not play nice with manual
transactions outside atomic blocks
Travis: Add postgres and mysql to the 1.6 test matrix

Also drop 1.6 from allowed failures as it should now build properly.