Use plugin-babel-istanbul for codecoverage

.babelrc

@@ -3,7 +3,7 @@
   "plugins": ["transform-runtime"],
   "env": {
     "test": {
-      "sourceMaps": "inline"
+      "plugins": ["istanbul"]
     }
   }
 }

package.json

@@ -18,6 +18,7 @@
     "ava": "^0.15.2",
     "babel-cli": "^6.0.0",
     "babel-helper-fixtures": "^6.6.5",
+    "babel-plugin-istanbul": "^1.0.3",
     "babel-plugin-transform-class-properties": "^6.6.0",
     "babel-plugin-transform-runtime": "^6.0.0",
     "babel-preset-es2015": "^6.0.0",
@@ -42,5 +43,9 @@
     "test": "npm run build && npm run lint && ava test/",
     "test_cov": "cross-env BABEL_ENV=test npm run build && npm run lint && nyc ava test/",
     "watch": "babel src --out-dir lib --watch"
+  },
+  "nyc": {
+    "sourceMap": false,
+    "instrument": false
   }
 }