Fixed minimum go version needed to 1.6.3 in readme

Fix #157

Signed-off-by: Cristian Maglie <c.maglie@arduino.cc>

Author: cmaglie

README.md

@@ -55,7 +55,7 @@ See [Doing continuous integration with arduino builder](https://github.com/ardui
 
 ### Building from source
 
-You need [Go 1.5.4](https://golang.org/dl/#go1.5.4).
+You need [Go 1.6.3](https://golang.org/dl/#go1.6.3).
 
 Repo root contains the script `setup_go_env_vars`. Use it as is or as a template for setting up Go environment variables.