Revert "Try to fix travis build on PHP < 7"

This reverts commit 779b687415d4861d2127db3fc425620d1cc7a5f8.

.travis.yml

@@ -107,11 +107,6 @@ install:
         # disable xdebug for performance reasons when code coverage is not needed. note: xdebug on hhvm is disabled by default
         phpenv config-rm xdebug.ini || echo "xdebug is not installed"
     fi
-  - |
-    if [[ $TRAVIS_PHP_VERSION < 7 ]]; then
-        # install GD
-        sudo apt-get install php$(phpenv version-name)-gd
-    fi
   - travis_retry composer self-update && composer --version
   - travis_retry composer global require "fxp/composer-asset-plugin:^1.2.0" --no-plugins
   - export PATH="$HOME/.composer/vendor/bin:$PATH"