Don't explicitly depend on astroid

we only depend on pylint, which in turn is responsible for
fetching astroid.

Author: atodorov

setup.py

@@ -27,7 +27,7 @@
     author_email='[email protected]',
     description=_short_description,
     version=_version,
-    install_requires=['pylint', 'astroid'],
+    install_requires=['pylint'],
     packages=_packages,
     license='GPLv2',
     classifiers=_classifiers,