Update README.rst

Now that django-polymorphic no longer supports 1.6,1.7,1.9, so removing these from readme also removing 3.3 Python. Based on the changes in this travis yml in this commit django-polymorphic@0c30a95#diff-354f30a63fb0907d4ad57269548329e3

Author: simkimsia

README.rst

@@ -182,12 +182,11 @@ To run the included test suite, execute::
 To test support for multiple Python and Django versions, you need to follow steps below:
 
 * install project requirements in virtual environment
-* install python 2.7, 3.3, 3.4, 3.5, 3.6 python versions through pyenv (See pyenv (Linux) or Homebrew (Mac OS X).)
+* install python 2.7, 3.4, 3.5, 3.6 python versions through pyenv (See pyenv (Linux) or Homebrew (Mac OS X).)
 * create .python-version file and add full list of installed versions with which project have to be tested, example::
 
  2.6.9
  2.7.13
- 3.3.6
  3.4.5
  3.5.2
  3.6.0
@@ -196,7 +195,7 @@ To test support for multiple Python and Django versions, you need to follow step
  pip install tox
  tox
 
-Python 2.7, 3.3, 3.4, 3.5 and 3.6 and django 1.7, 1.8, 1.9, 1.10 and 1.11 are the currently supported versions.
+Python 2.7, 3.4, 3.5 and 3.6 and django 1.8, 1.10 and 1.11 are the currently supported versions.
 
 Todo
 ----