opencensus-ext-django - support all versions of django 1.11

[rib3]

opencensus-ext-django: Support all releases of django 1.11

1.11.21 was just released, which is not compatible with the current install_requires
https://www.djangoproject.com/weblog/2019/jun/03/security-releases/

[songy23]

LGTM

[rib3]

I tweaked the comparison since it appears that distutils handles alpha,beta,rc safely when specifying patch version numbers

     include_package_data=True,
     long_description=open('README.rst').read(),
     install_requires=[
-        'Django >= 1.11.0, < 1.12',
+        'Django >= 1.11.0, < 1.12.0',
         'opencensus >= 0.7.dev0, < 1.0.0',
     ],
     extras_require={},

from distutils.version import LooseVersion
In [1]: from distutils.version import LooseVersion

In [2]: LooseVersion('1.12rc1') < LooseVersion('1.12.0')
Out[2]: False

In [3]: LooseVersion('1.12alpha1') < LooseVersion('1.12.0')
Out[3]: False

In [4]: LooseVersion('1.12beta1') < LooseVersion('1.12.0')
Out[4]: False

[reyang]

@rib3 Thanks for the fix!

@zoidbergwill FYI since it is related with #358.