Lower minimum dependency

Right now you are manually including API Level 15, but most of the functionality you're relying on has been present in Android since before that (at least up until API Level 10, where RXJava is supported). Previously, you would receive an error message like this if attempting to use this library on an earlier version;

```
ERROR: Debug has an indirect dependency on Android API level 15, but minSdkVersion for variant 'Debug' is API level 10
```

After this change, I was able to build a project and run it on an Android 2.3.7 device properly.

Keep up the great work! This library shows a ton of promise.