update image used for build-without-gcc step (#4818)

As we've dropped support for go 1.8 (see 1.5.0 release post),
now we need to ensure that a minimum of go 1.9 builds correctly.

.drone.yml

@@ -56,7 +56,7 @@ pipeline:
       event: [ push, tag, pull_request ]
 
   build-without-gcc:
-    image: golang:1.8
+    image: golang:1.9
     pull: true
     commands:
       - go build -o gitea_no_gcc # test if build succeeds without the sqlite tag