Back to "replace" for ext-mongo

To use this package, either composer.json or any dependency needs to require "ext-mongo", see https://github.com/composer/composer/issues/2690 and https://github.com/alcaeus/mongo-php-adapter/issues/67 and https://github.com/heroku/heroku-buildpack-php/issues/146

composer.json

@@ -16,7 +16,7 @@
     "require-dev": {
         "phpunit/phpunit": "^4.8 || ^5.0"
     },
-    "provide": {
+    "replace": {
         "ext-mongo": "1.6.12"
     },
     "autoload": {