Add supported python versions to readme

Given that PEP 0414 was only implemented on 3.3, Python 3
of version 3.2 and prior is not supported, as it would be
cumbersome to add, and 3.2 is used by few anyway.

.travis.yml

@@ -1,8 +1,6 @@
 language: python
 python:
-  - "2.6"
   - "2.7"
-  - "3.2"
   - "3.3"
   - "3.4"
   - "3.5"

readme.md

@@ -10,8 +10,10 @@ Pubs is built with the following principles in mind:
  - bibliographic data (i.e. pure bibtex information) is kept separated from metadata (including links to pdf or tags),
  - everything is stored in plain text so it can be manually edited or version controlled.
 
+Pubs is compatible with Python 2.7 and 3.3 and higher.
+
+**Notice:** pubs is still in early development and cannot be considered as stable.
 
-**Notice:** pubs is still in early development and cannot be considered as stable
 
 
 Getting started