Don't install README.txt as a data file.

The change in d5d1891 means systems packages
end up with a file in sys.prefix, such as /usr/README.txt.

setup.py

@@ -7,7 +7,6 @@
       author='Gary Bernhardt',
       author_email='gary.bernhardt@gmail.com',
       py_modules=['dingus'],
-      data_files=[('', ['README.txt'])],
       license='MIT',
       url='https://github.com/garybernhardt/dingus',
       keywords='testing test mocking mock double stub fake record assert',