travis-ci: install babel with pip to stop easy_install

easy_install just fails, trying to install into ./.eggs
https://travis-ci.org/fdroidtravis/fdroidserver/jobs/319144754
2 jobs for nail-down-tests in 26 minutes and 36 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #45412694
metadata_v0

00:04:04

passed #45412693
test

00:26:36