Revert ""providing" ext-bcmath breaks Heroku"

This reverts commit f805922.

Author: terrafrost

composer.json

@@ -42,5 +42,8 @@
         "test": "phpunit",
         "check-style": "phpcs src tests",
         "fix-style": "phpcbf src tests"
+    },
+    "provide": {
+      "ext-bcmath": "7.3.5"
     }
 }