Travis should prefer dist.

Author: beberlei

.travis.yml

@@ -16,7 +16,7 @@ before_script:
   - sh -c "if [ '$DB' = 'pgsql' ]; then psql -c 'create database doctrine_tests;' -U postgres; fi"
   - sh -c "if [ '$DB' = 'pgsql' ]; then psql -c 'create database doctrine_tests_tmp;' -U postgres; fi"
   - sh -c "if [ '$DB' = 'mysql' ]; then mysql -e 'create database IF NOT EXISTS doctrine_tests_tmp;create database IF NOT EXISTS doctrine_tests;'; fi"
-  - composer install --prefer-source --dev
+  - composer install --prefer-dist --dev
 
 script: phpunit --configuration tests/travis/$DB.travis.xml