Attempt to fix travis issue

Per friends-of-phpspec/phpspec-code-coverage#42 (comment)

Author: eileenmcnaughton

.travis.yml

@@ -9,6 +9,7 @@ php:
 
 before_script:
   - composer install -n --dev --prefer-source
+  - export XDEBUG_MODE=coverage
 
 script:
   - vendor/bin/phpcs --standard=PSR2 src