traceback when Jira ticket is linked to a TestCase

[kkaarreell]

The code expects only bugzilla bugs are linked to TCs. If there is a Jira issue with ID FOO-123 linked, python-nitrate tracebacks

list(nitrate.TestCase(12345).bugs)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.6/site-packages/nitrate/containers.py", line 155, in __len__
    return len(self._items)
  File "/usr/lib/python3.6/site-packages/nitrate/containers.py", line 105, in _items
    self._fetch()
  File "/usr/lib/python3.6/site-packages/nitrate/containers.py", line 411, in _fetch
    self._current = set([Bug(inject) for inject in injects])
  File "/usr/lib/python3.6/site-packages/nitrate/containers.py", line 411, in <listcomp>
    self._current = set([Bug(inject) for inject in injects])
  File "/usr/lib/python3.6/site-packages/nitrate/immutable.py", line 1191, in __init__
    self._fetch(inject)
  File "/usr/lib/python3.6/site-packages/nitrate/immutable.py", line 1237, in _fetch
    self._bug = int(inject["bug_id"])
ValueError: invalid literal for int() with base 10: 'FOO-123'

[psss]

Should be fixed by #35.