Share common requirements for Travis and distribution (fix #319)

Requirements that are shared for normal installations and Travis
tests are in .travis/requirements.txt. Other requirements are
specified in requirements.txt for normal installations and
.travis.yml for Travis tests.

.travis/requirements.txt

-# Common denominators for testing
-Markdown>=2.2.0
-django-sekizai>=0.7
+# Django requirement is set in .travis.yml or requirements.txt
+
+# sorl-thumbnail requirement is set in .travis.yml or requirements.txt
 
-# Removed for now because it is not universal as sorl is broken on
-# django 1.4: https://github.com/mariocesar/sorl-thumbnail/issues/255
-# sorl-thumbnail>=11.12.1b
+# Wiki language processor
+Markdown>2.2.0
+
+# Package for flexible block handling
+django-sekizai>=0.7
 
-# Switch to Pillow and the dev version of sorl v12
+# Imaging library
 Pillow
+
 # Notification system
-django-nyt>=0.9.1
-# django-mptt 0.6.1 is broken, see #316
+django-nyt>=0.9.3
+
+# Tree structure handling
+# django-mptt 0.6.1 is broken, see
+# https://github.com/django-mptt/django-mptt/issues/316
 django-mptt==0.6.0
+
+# Python 2 and 3 compatibility
 six
 

requirements.txt

+# The web framework
 django>=1.4,<1.7
-Markdown>2.2.0
-django-sekizai>=0.7
 
-# django-mptt 0.6.1 is broken, see #316
-django-mptt==0.6.0
-
-# Now with compatibility for django 1.6
+# Thumbnails in Django
+# For Django 1.4, use sorl-thumbnail<11.12.1:
+# https://github.com/mariocesar/sorl-thumbnail/issues/255
 sorl-thumbnail>=11.12.1b
 
-Pillow
-six
-# Notification system
-django-nyt>=0.9.3
+# Migrations in Django
+South>=0.8.4
+
+# Use requirements that are common with Travis tests
+-r .travis/requirements.txt