Fix coverage reports

Using nyc to instrument the files lead to false negatives, not to mention
incorrect call count for lines in the HTML report. Switched to
babel-plugin-istanbul.

Had to move the `excluded` stanza from `.nycrc` to `.babelrc` because of
istanbuljs/babel-plugin-istanbul#62

.babelrc

@@ -5,6 +5,16 @@
   "env": {
     "development": {
       "presets": ["react-hmre"]
+    },
+
+    "tests": {
+      "plugins": [
+        ["istanbul", {
+          "exclude": [
+            "tests/unit/**/*.js"
+          ]
+        }]
+      ]
     }
   }
 }

.nycrc

@@ -3,10 +3,11 @@
     "src/**/*.js",
     "src/**/*.jsx"
   ],
-  "exclude": [
-    "tests/unit/**/*.js"
-  ],
   "extension": [".js", ".jsx"],
+
+  "sourceMap": false,
+  "instrument": false,
+
   "reporter": ["html"],
 
   "lines": 100,

package.json

@@ -50,6 +50,7 @@
     "babel-core": "~6.22.1",
     "babel-eslint": "~7.1.1",
     "babel-loader": "~6.2.4",
+    "babel-plugin-istanbul": "~3.1.2",
     "babel-plugin-transform-class-properties": "~6.9.0",
     "babel-plugin-transform-object-rest-spread": "~6.20.2",
     "babel-preset-es2015": "~6.9.0",