README.rst: Updated coala banner

The banner on README.rst has been updated to match with that of https://github.com/coala/coala
Closes coala#436

Author: yashasingh

README.rst

@@ -1,4 +1,5 @@
-.. image:: https://cloud.githubusercontent.com/assets/7521600/15992701/ef245fd4-30ef-11e6-992d-275c5ca7c3a0.jpg
+.. image:: https://cloud.githubusercontent.com/assets/5716520/24838296/a9cf5f04-1d45-11e7-855c-47b816ce1e09.png
+    :target: https://coala.io/
 
 The official documentation for coala
 ====================================