Use babel-plugin-dev-expression

This will automatically do the right thing with our __DEV__ statements
that we use to prune out development-only code from our production
builds.

Helps #111
Fixes #112

.size-snapshot.json

@@ -1,26 +1,26 @@
 {
   "esm/react-media.js": {
-    "bundled": 3856,
-    "minified": 2318,
-    "gzipped": 885,
+    "bundled": 3965,
+    "minified": 2402,
+    "gzipped": 918,
     "treeshaked": {
       "rollup": {
-        "code": 1928,
-        "import_statements": 272
+        "code": 1653,
+        "import_statements": 265
       },
       "webpack": {
-        "code": 3186
+        "code": 2885
       }
     }
   },
   "umd/react-media.js": {
-    "bundled": 34826,
-    "minified": 10989,
-    "gzipped": 3946
+    "bundled": 34833,
+    "minified": 10982,
+    "gzipped": 3944
   },
   "umd/react-media.min.js": {
-    "bundled": 11533,
-    "minified": 4294,
-    "gzipped": 1936
+    "bundled": 11106,
+    "minified": 4014,
+    "gzipped": 1837
   }
 }

modules/.babelrc

@@ -2,6 +2,7 @@
   "presets": [["@babel/env", { "loose": true }], "@babel/react"],
   "plugins": [
     "@babel/proposal-class-properties",
-    "@babel/proposal-export-default-from"
+    "@babel/proposal-export-default-from",
+    "dev-expression"
   ]
 }