dropbox: Only support on go1.7+

See https://github.com/dropbox/dropbox-sdk-go-unofficial/pull/40

backend/dropbox/dropbox.go

@@ -1,4 +1,7 @@
 // Package dropbox provides an interface to Dropbox object storage
+
+// +build go1.7
+
 package dropbox
 
 // FIXME dropbox for business would be quite easy to add

backend/dropbox/dropbox_test.go

@@ -2,6 +2,9 @@
 //
 // Automatically generated - DO NOT EDIT
 // Regenerate with: make gen_tests
+
+// +build go1.7
+
 package dropbox_test
 
 import (

backend/dropbox/dropbox_unsupported.go

@@ -0,0 +1,6 @@
+// Build for unsupported platforms to stop go complaining about "no
+// buildable Go source files "
+
+// +build !go1.7
+
+package dropbox

fstest/fstests/gen_tests.go

@@ -150,7 +150,7 @@ func main() {
 	generateTestProgram(t, fns, "S3")
 	generateTestProgram(t, fns, "Drive")
 	generateTestProgram(t, fns, "GoogleCloudStorage")
-	generateTestProgram(t, fns, "Dropbox")
+	generateTestProgram(t, fns, "Dropbox", buildConstraint("go1.7"))
 	generateTestProgram(t, fns, "AmazonCloudDrive")
 	generateTestProgram(t, fns, "OneDrive")
 	generateTestProgram(t, fns, "Hubic")