Merge branch 'hotfix/travis-composer'

Close #86

Author: weierophinney

.travis.yml

@@ -32,7 +32,7 @@ before_install:
   - if [[ $EXECUTE_COVERAGE == 'true' ]]; then composer require --no-update satooshi/php-coveralls:dev-master ; fi
 
 install:
-  - travis_retry composer install --no-interaction --ignore-platform-reqs
+  - travis_retry composer install --no-interaction --ignore-platform-reqs --prefer-source
   - composer info -i
 
 script: