Don't run `composer --version` on install or before_install

Parsedown does not require any dependecies installed via Composer to run the tests.
Travis CI is automatically running `composer --version`.
But on PHP 5.2 Composer is not working.
No need to run `composer --version` on `before_install` or `install` phase.

`true` is a *nix command which exists with 0 and therefore disables the default Travis behavior.