set travis composer cache + update readme

Author: tiptopcoder

.travis.yml

@@ -58,3 +58,8 @@ script:
     else
       vendor/bin/phpunit;
     fi
+
+cache:
+  directories:
+    - vendor
+    - "$HOME/.composer/cache"

README.md

@@ -1,5 +1,7 @@
 # Google Auth Library for [Firebase PHP](https://github.com/firebase-php/firebase-php)
 
+[![Build Status](https://travis-ci.org/firebase-php/google-auth-library-php.svg?branch=master)](https://travis-ci.org/firebase-php/google-auth-library-php)
+
 > This is a fork from [Google Auth Library for PHP](https://github.com/googleapis/google-auth-library-php) which is internally used in [Firebase PHP](https://github.com/firebase-php/firebase-php)
 
 ### What's changed