Update soong's go.mod to 1.15.6

Test: TH
Change-Id: I370e4a260cb6fb9fbd2d2da44964b51a339b90f5
diff --git a/go.mod b/go.mod
index 7297dea..117fa65 100644
--- a/go.mod
+++ b/go.mod
@@ -8,4 +8,4 @@
 
 replace github.com/google/blueprint v0.0.0 => ../blueprint
 
-go 1.15
+go 1.15.6