0.7.12 Django integration duplicates DB entries

[shabble]

In opencensus-ext-django==0.7.12, the following setup line has somehow gotten doubled up, producing duplicate output in traces.

opencensus-python/contrib/opencensus-ext-django/opencensus/ext/django/middleware.py

Line 167 in ea40da8

if django.VERSION >= (2,): # pragma: NO COVER

Looks like it might be a result of the merge in #995 of #775, when the merge destination already had that code present.

[lzchen]

Should be fixed with https://pypi.org/project/opencensus-ext-django/0.7.4/