Add require-dev in order to remove downloads of phars.
Use vendor/bin
No need for behat through ci/perpare.sh
No behat @dev versions.
Merge pull request #1026 from clemens-tolboom/feature/require-dev

Add require-dev in order to remove downloads of phars.
make phpunit & behat paths more tab-completion friendly
wp eval-file: mention $args and add example

see #1014