setup.py: requirements: Change "dateutils" to "dateutil".

setup.py

@@ -6,7 +6,7 @@ setup(name='Mastodon.py',
       packages=['mastodon'],
       setup_requires=['pytest-runner'],
       tests_require=['pytest'],
-      install_requires=['requests', 'dateutils', 'six'],
+      install_requires=['requests', 'dateutil', 'six'],
       url='https://github.com/halcy/Mastodon.py',
       author='Lorenz Diener',
       author_email='lorenzd+mastodonpypypi@gmail.com',