doctrine/mongodb 1.4.x-dev requires ext-mongo ^1.5
ext-mongo is the legacy MongoDB driver (\MongoClient). ext-mongo was never ported to Zend 3 (PHP7). The new Mongo driver that does work under PHP7 (the one you installed via PECL) is ext-mongodb. This is not supported by the Doctrine driver for MongoDB.
You have two choices: find a Doctrine driver that does work with the newer Mongo extension, or downgrade to PHP 5.6 and install ext-mongo.