Replace travis build matrix with stages
Replace travis build matrix with stages

With this change, the build first runs a single smoke screen build against the latest versions to save on build resources. Remaining topologies, server versions and PHP versions will be tested in parallel if this first build step was successful.

10gen-labs/mongo-php-library-prototype,jmikola/mongo-php-library,jmikola/mongo-php-library-prototype,jmikola/mongo-php-library,jmikola/mongo-php-library,mongodb/mongo-php-library,mongodb/mongo-php-library,mongodb/mongo-php-library,jmikola/mongo-php-library,mongodb/mongo-php-library