add integration test that fetches and compiles code using go 1.11

test/README.md

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+This directory contains integration tests that check that go-arg can be installed and compiled under both tgo 1.10 and go 1.11 (corresponding to the transition to the new go module system)

test/compile_with_go111.sh

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+#!/bin/bash
+
+docker run \
+    --rm \
+    -v $(pwd)/some-program:/src \
+    -w /src \
+    golang:1.11 \
+    go build

test/some-program/go.mod

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+module some-program
+
+require github.com/alexflint/go-arg v1.0.0

test/some-program/go.sum

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+github.com/alexflint/go-arg v1.0.0 h1:VWNnY3DyBHiq5lcwY2FlCE5t5qyHNV0o5i1bkCIHprU=
+github.com/alexflint/go-arg v1.0.0/go.mod h1:Cto8k5VtkP4pp0EXiWD4ZJMFOOinZ38ggVcQ/6CGuRI=
+github.com/alexflint/go-scalar v1.0.0 h1:NGupf1XV/Xb04wXskDFzS0KWOLH632W/EO4fAFi+A70=
+github.com/alexflint/go-scalar v1.0.0/go.mod h1:GpHzbCOZXEKMEcygYQ5n/aa4Aq84zbxjy3MxYW0gjYw=
+github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/kr/pretty v0.1.0/go.mod h1:dAy3ld7l9f0ibDNOQOHHMYYIIbhfbHSm3C4ZsoJORNo=
+github.com/kr/pty v1.1.1/go.mod h1:pFQYn66WHrOpPYNljwOMqo10TkYh1fy3cYio2l3bCsQ=
+github.com/kr/text v0.1.0/go.mod h1:4Jbv+DJW3UT/LiOwJeYQe1efqtUx/iVham/4vfdArNI=
+github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/stretchr/testify v1.2.2/go.mod h1:a8OnRcib4nhh0OaRAV+Yts87kKdq0PP7pXfy6kDkUVs=

test/some-program/main.go

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+package main
+
+import "github.com/alexflint/go-arg"
+
+func main() {
+	var args struct {
+		Test string
+	}
+	arg.MustParse(&args)
+}