Add Travis CI Build Matrix to decrease testing time

 - This PR adds [Build Matrix](http://docs.travis-ci.com/user/build-configuration/#The-Build-Matrix) to decrease testing time.
    - You can see [my build log](https://travis-ci.org/t-mat/lz4/builds/24114276).
 - And `dev` branch's [status image](http://docs.travis-ci.com/user/status-images/) to README.md
    - Here is an example : [![Build Status](https://travis-ci.org/Cyan4973/lz4.svg?branch=dev)](https://travis-ci.org/Cyan4973/lz4)