Fix failing builds on TravisCI

Wrap apt-get update && apt-get install into travis_retry

.travis.yml

@@ -246,8 +246,8 @@ before_script:
 
 
   - echo "Setting up Apache + FPM"
-  - sudo apt-get update
-  - sudo apt-get install apache2 libapache2-mod-fastcgi
+  - travis_retry sudo apt-get update
+  - travis_retry sudo apt-get install apache2 libapache2-mod-fastcgi
   # enable php-fpm
   - sudo cp ~/.phpenv/versions/$(phpenv version-name)/etc/php-fpm.conf.default ~/.phpenv/versions/$(phpenv version-name)/etc/php-fpm.conf
   - |