Add shields

Add shields for Travis-CI and Maven Central.

Preview:
[![Build Status](http://img.shields.io/travis/BoltsFramework/Bolts-Android/master.svg?style=flat)](https://travis-ci.org/BoltsFramework/Bolts-Android) [![Maven Central](https://maven-badges.herokuapp.com/maven-central/com.parse.bolts/bolts-android/badge.svg?style=flat)](https://maven-badges.herokuapp.com/maven-central/com.parse.bolts/bolts-android)