Fix travis configuration

Author: enumag

.travis.yml

@@ -27,7 +27,7 @@ install:
     - travis_retry composer update --no-interaction --prefer-source
 
 before_script:
-    - if [[ $TRAVIS_PHP_VERSION != hhvm* ]]; then coverage="--coverage-xml"; fi
+    - if [[ $TRAVIS_PHP_VERSION != hhvm* ]] && [[ $TRAVIS_PHP_VERSION != *nightly ]]; then coverage="--coverage-xml"; fi
     - vendor/bin/codecept build
 
 script: