add test that compiles under go 1.11 without the module system

test/compile_with_go111.sh

@@ -5,4 +5,4 @@ docker run \
     -v $(pwd)/some-program:/src \
     -w /src \
     golang:1.11 \
-    go build
+    go build -o /dev/null

test/compile_with_go111_inside_gopath.sh

@@ -0,0 +1,12 @@
+#!/bin/bash
+
+# Under go 1.11, modules are disabled by default when user code is located within the GOPATH.
+# In this test, we check that we can correctly "go get" the go-arg package, and then compile
+# some code that uses it.
+
+docker run \
+    --rm \
+    -v $(pwd)/some-program:/go/src/some-program \
+    -w /go/src/some-program \
+    golang:1.11 \
+    bash -c "go get github.com/alexflint/go-arg && go build"