setup.py: requirements => install_requires

Fixed one small typo. This was causing a warning and to make the package not depend on `six` (which I caught when my Travis CI build failed! https://travis-ci.org/msabramo/python-memcached/jobs/21531412)

```
 ❯ python setup.py egg_info
/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/distutils/dist.py:267: UserWarning: Unknown distribution option: 'requirements'
  warnings.warn(msg)
running egg_info
writing python_memcached.egg-info/PKG-INFO
writing top-level names to python_memcached.egg-info/top_level.txt
writing dependency_links to python_memcached.egg-info/dependency_links.txt
reading manifest template 'MANIFEST.in'
warning: no files found matching '*.rst'
warning: no files found matching 'MakeFile'
warning: no previously-included files matching '*.pyc' found anywhere in distribution
warning: no previously-included files matching '.DS_Store' found anywhere in distribution
writing manifest file 'python_memcached.egg-info/SOURCES.txt'
```