Correct requirements list

Includes decorator and six in the README, and also updates setup.py to ensure
we have a good version of six.

Closes: #86

Author: frozencemetery

README.txt

@@ -34,6 +34,8 @@ Basic
 
 * either the `enum34` Python package or Python 3.4+
 
+* the `six` and `decorator` python packages
+
 Compiling from Scratch
 ----------------------
 
@@ -59,6 +61,8 @@ Easy Way
 From the Git Repo
 -----------------
 
+After being sure to install all the requirements,
+
 .. code-block:: bash
 
     $ git clone https://github.com/pythongssapi/python-gssapi.git

setup.py

@@ -208,7 +208,7 @@ def gssapi_modules(lst):
 
 install_requires = [
     'decorator',
-    'six'
+    'six >= 1.4.0'
 ]
 if sys.version_info < (3, 4):
     install_requires.append('enum34')