Add PIL to requirements.txt and remove Python 2.5 from travis

.travis.yml

 language: python
 python:
-  - "2.5"
   - "2.6"
   - "2.7"
 # command to run tests

.travis/requirements.txt

@@ -3,4 +3,4 @@ Markdown<2.3.0
 django-sekizai<0.7
 django-mptt>=0.5.3
 sorl-thumbnail
-
+PIL

requirements.txt

@@ -4,3 +4,4 @@ Markdown<2.3.0
 django-sekizai<0.7
 django-mptt>=0.5.3
 sorl-thumbnail
+PIL