Sync Transifex config with the source lang change in Transifex

Closes-Bug: #1179526

source_lang mismatch between Horizon and OpenStack Hub projects
has been addressed and the soruce language is changed from en_US
to en. This commit updates .tx/config to catch up it.

Also adds an entry for djangojs.po translation which was added
to Transifex recently.

Change-Id: I9a574b92a8396d2fda493d8dfa1261ddccc4ad02