Upgrade meteor-promise to version 0.8.4.

This is a change that was necessary on the wip-upgrade-to-node-6 branch,
and it seems better to ship it sooner rather than waiting:
meteor/meteor@d823812e85

scripts/dev-bundle-server-package.js

@@ -9,7 +9,7 @@ var packageJson = {
   // Version is not important but is needed to prevent warnings.
   version: "0.0.0",
   dependencies: {
-    "meteor-promise": "0.8.0",
+    "meteor-promise": "0.8.4",
     fibers: "1.0.15",
     promise: "7.1.1",
     // Not yet upgrading Underscore from 1.5.2 to 1.7.0 (which should be done

scripts/dev-bundle-tool-package.js

@@ -16,7 +16,7 @@ var packageJson = {
     "node-pre-gyp": "0.6.34",
     "meteor-babel": "0.21.4",
     reify: "0.11.21",
-    "meteor-promise": "0.8.0",
+    "meteor-promise": "0.8.4",
     fibers: "1.0.15",
     promise: "7.1.1",
     // So that Babel 6 can emit require("babel-runtime/helpers/...") calls.