Composer 2.0 compatibility
Composer 2.0 compatibility (#62) * Update composer.json * Support both Composer versions 1 and 2 * Replace deprecated constants * Try to run on Travis with both Composer 1.x and 2.x versions * Try to run on Travis with both Composer 1.x and 2.x versions * Try to run on Travis with both Composer 1.x and 2.x versions * Add askForUpdate for composer 2.0