Stick to react-native-background-timer 1.0.0

Recently expose-loader broke us when it updated from 0.7.1 to 0.7.2 and
we decided to stick to exact versions.

package.json

     "react": "15.4.2",
     "react-dom": "15.4.2",
     "react-native": "0.41.2",
-    "react-native-background-timer": "^1.0.0",
+    "react-native-background-timer": "1.0.0",
     "react-native-immersive": "0.0.4",
     "react-native-keep-awake": "^2.0.2",
     "react-native-prompt": "^1.0.0",