⬆️ 🔨 Upgrade PHP Markdown library.

The current version throws deprecated warning with PHP7.1 and PHPUnit.
Upgrade the Markdown library to the current PHP Markdown Lib 1.7.0.
Used composer to manage this library.

.travis.yml

@@ -32,7 +32,7 @@ matrix:
 #  - if [[ "$DB" == "mysql" ]]; then mysql -e "create database IF NOT EXISTS hello_world_test;" -uroot; fi
 
 install:
-  - composer require phpunit/phpunit
+  - composer install --optimize-autoloader
 
 # omitting "script:" will default to phpunit
 # use the $DB env variable to determine the phpunit.xml to use