Pin Flask-WTF version to 0.8.4

Flask-WTF has had a couple of new releases recently, breaking the current code.

```pytb
(.venv)joe@joe-ideapad:~/git/notifico (master)$ python notifico/
Traceback (most recent call last):
  File "/usr/lib/python2.7/runpy.py", line 162, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/usr/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/home/joe/git/notifico/notifico/__main__.py", line 19, in <module>
    from notifico.models import *
  File "/home/joe/git/notifico/notifico/models/__init__.py", line 13, in <module>
    from notifico.models.hook import *
  File "/home/joe/git/notifico/notifico/models/hook.py", line 8, in <module>
    from notifico.services.hooks import HookService
  File "/home/joe/git/notifico/notifico/services/hooks/__init__.py", line 3, in <module>
    from notifico.services.hooks.bitbucket import *
  File "/home/joe/git/notifico/notifico/services/hooks/bitbucket.py", line 11, in <module>
    class BitbucketConfigForm(wtf.Form):
  File "/home/joe/git/notifico/notifico/services/hooks/bitbucket.py", line 12, in BitbucketConfigForm
    branches = wtf.TextField('Branches', validators=[
AttributeError: 'module' object has no attribute 'TextField'

```