README: git checkout 1.6.0 in the instructions.

(cherry picked from commit c6d5c40)

Author: fredrikekre

README.md

@@ -81,7 +81,7 @@ Julia. However, most users should use the most recent stable version
 of Julia. You can get this version by changing to the Julia directory
 and running:
 
-    git checkout v1.5.0
+    git checkout v1.6.0
 
 Now run `make` to build the `julia` executable.