Added PHP 5.5 on travis and forced the source install

The dist packages are less reliable on github.

Author: stof

.travis.yml

@@ -3,8 +3,9 @@ language: php
 php:
   - 5.3
   - 5.4
+  - 5.5
 
-before_script: composer install --dev
+before_script: composer install --dev --prefer-source
 
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