Fix inclusion of tests package in setup.py (#131)

diff --git a/setup.py b/setup.py
index 5f72e00..389daff 100644
--- a/setup.py
+++ b/setup.py
@@ -39,7 +39,7 @@
     description='Google Authentication Library',
     long_description=long_description,
     url='https://github.com/GoogleCloudPlatform/google-auth-library-python',
-    packages=find_packages(exclude=('tests', 'system_tests')),
+    packages=find_packages(exclude=('tests*', 'system_tests*')),
     namespace_packages=('google',),
     install_requires=DEPENDENCIES,
     extras_require={