Add PHPUnit as a dev dependency in Composer

We may have previously discussed this, but there is still
no strong argument against having a PHPUnit as a dev dependency.

[The relevant
documentation](http://getcomposer.org/doc/04-schema.md#require-dev)

* PHPUnit is required to run the tests, so according to the Composer
documentation it should be there.
* It will be installed only when running `composer install` in the Parsedown
folder and not for users installing Parsedown as a dependency.
* It won't slow down the Travis CI build, because it is not running `composer
install` ATM.

I have commited the `composer.lock` as well, but [it is optional for libraries](http://getcomposer.org/doc/02-libraries.md#lock-file).
It is just my preference, so I could ammend my commit if you prefer the opposite.