Include setuptools as dependency (#31)

It is needed to provide pkg_resources

setup.py

@@ -8,7 +8,7 @@
       author='Danilo Bargen',
       author_email='mail@dbrgn.ch',
       url='https://github.com/dbrgn/coverage-badge',
-      install_requires=['coverage'],
+      install_requires=['coverage', 'setuptools'],
       packages=['coverage_badge'],
       zip_safe=True,
       include_package_data=True,