drop --dev switch in composer install step

`--dev` packages are installed by default. This fixes a composer warning.

Author: staabm

.travis.yml

@@ -8,7 +8,7 @@ php:
     - 5.6
 
 before_script:
-    - COMPOSER_ROOT_VERSION=dev-master composer install --dev --prefer-source
+    - COMPOSER_ROOT_VERSION=dev-master composer install --prefer-source
 
 script: vendor/bin/phpunit --configuration ./build/travis-ci.xml