Removed PHP v5.3 from matrix and added "--prefer-source" argument for…

… "composer install" command.

Signed-off-by: Florian Wolters <[email protected]>

.travis.yml

@@ -23,7 +23,6 @@
 language: php
 
 php:
-  - 5.3
   - 5.4
   - 5.5
   - 5.6
@@ -35,7 +34,7 @@ matrix:
 
 before_script:
   - composer selfupdate
-  - composer install
+  - composer install --prefer-source
   - wget https://scrutinizer-ci.com/ocular.phar
 
 script: