Add extension mbstring for test

Author: vonglasow

.travis.yml

@@ -44,6 +44,7 @@ cache:
 
 before_script:
   - export PATH="$PATH:$HOME/.composer/vendor/bin"
+  - echo 'extension = mbstring.so' >> ~/.phpenv/versions/$(phpenv version-name)/etc/conf.d/travis.ini
   - if [[ ! $ENABLE_XDEBUG ]]; then
       phpenv config-rm xdebug.ini || echo "ext-xdebug is not available, cannot remove it.";
     fi