Fix typo in appveyor.yml "evrsion" -> "version" (pypa#274)

Author: nicholasbishop

source/code/appveyor.yml

@@ -31,7 +31,7 @@ test_script:
   # only needed to support those cases.
   # Note that you must use the environment variable %PYTHON% to refer to
   # the interpreter you're using - Appveyor does not do anything special
-  # to put the Python evrsion you want to use on PATH.
+  # to put the Python version you want to use on PATH.
   - "build.cmd %PYTHON%\\python.exe setup.py test"
 
 after_test: