improve the Travis CI config

[xabbuh]

• copying the PHPUnit config is not necessary, PHPUnit will use the dist file by default anyway
• caching Composer files will speed up successive builds
• the --dev option is set by default on composer install
• disable the Xdebug extension to speed up the test execution
• use --prefer-dist instead of --prefer-source to speed up package installation

[xabbuh]

see also https://docs.travis-ci.com/user/speeding-up-the-build/ and https://getcomposer.org/doc/03-cli.md#install-i

[jschaedl]

Thanks @xabbuh !