Simplify object check and eliminate lodash depdency (reduxjs#936)

* Simplify object check and eliminate lodash depdency, mirroring reduxjs/redux#2599 .

* Add missing files.

* Replace isPlainObject with unmodified version from Redux

* Also replace the spec with the unmodified version

Author: matthargett

.babelrc

@@ -34,14 +34,8 @@
         ["transform-es2015-modules-commonjs", { "loose": true }]
       ]
     },
-    "es": {
-      "plugins": [
-        "./build/use-lodash-es"
-      ]
-    },
     "rollup": {
       "plugins": [
-        "./build/use-lodash-es",
         "external-helpers"
       ]
     }