Revert "Use provide instead of replace in composer.json"

This reverts commit 87acb38369c77e916dda95e3903e87197b5a6703.

composer.json

@@ -15,7 +15,7 @@
     "require-dev": {
         "phpunit/phpunit": "^4.8 || ^5.0"
     },
-    "provide": {
+    "replace": {
         "ext-mongo": "1.6.12"
     },
     "autoload": {