lock form for the duration of form processing

this prevents another process from attempting to modify
the form between the first and second save
update dimagi-utils ref
add redis service to .travis
fix form edit (broken by form-locking)
remove use_locking option

instead fix tests to call correctly
and add util primarily for tests that posts and immediately releases the lock

(this new function is called post_xform_to_couch so that tests may continue to use;
the old function is renamed to create_and_lock_xform)
Merge pull request #98 from dimagi/form-locking

lock form for the duration of form processing