Fix TravisCI setup

Author: JeroenDeDauw

build/travis/before_script.sh

@@ -10,12 +10,10 @@ cd ..
 
 wget https://github.com/wikimedia/mediawiki-core/archive/$MW.tar.gz
 tar -zxf $MW.tar.gz
-mv $MW phase3
+mv mediawiki-$MW phase3
 
 cd phase3
 
-git checkout $MW
-
 travis_retry composer install --prefer-source
 
 if [ "$DB" == "postgres" ]