Update travis to run make build

Ensure all packages can be built, even those not yet imported by binaries.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

fs/fstest/file_unix.go

@@ -0,0 +1,11 @@
+// +build !windows
+
+package fstest
+
+import "github.com/containerd/continuity/sysx"
+
+func SetXAttr(name, key, value string) Applier {
+	return applyFn(func(root string) error {
+		return sysx.LSetxattr(name, key, []byte(value), 0)
+	})
+}