Explain python ver choice + add pypy

.travis.yml

@@ -6,15 +6,17 @@ addons:
     - redis-server
 
 python:
+  - "2.5"
   - "2.6"
   - "2.7"
-  # - "3.2"
+  # - "3.2" # humanfriendly cant install
   - "3.3"
   - "3.4"
   - "3.5"
-  - "3.5-dev" # 3.5 development branch
-  - "nightly" # currently points to 3.6-dev
-# command to install dependencies
+  - "3.5-dev"
+  - "nightly"
+  - "pypy"
+  - "pypy3"
 
 install: 
   - "pip install --upgrade -r requirements.txt"