Move pyside to travis using apt and added to external_dependencies.txt

As it needs to compile and install itself if done pip install, that errors and takes a lot of time in travis

.travis.yml

@@ -5,6 +5,7 @@ addons:
   apt:
     packages:
     - redis-server
+    - python-pyside
 
 python:
   # major versions, test first

external_dependencies.txt

@@ -0,0 +1,3 @@
+PySide
+  - On linux, sudo apt-get install python-pyside
+  - On Windows, install installers from their website

requirements.txt

@@ -1,7 +1,6 @@
 redis
 pytest
 htmlpy
-PySide
 
 python-dateutil
 humanfriendly