drive, googlecloudstorage: disallow on go1.8 due to dependent library changes

golang.org/x/oauth2/google no longer builds on go1.8

backend/drive/drive.go

@@ -1,4 +1,7 @@
 // Package drive interfaces with the Google Drive object storage system
+
+// +build go1.9
+
 package drive
 
 // FIXME need to deal with some corner cases

backend/drive/drive_internal_test.go

@@ -1,3 +1,5 @@
+// +build go1.9
+
 package drive
 
 import (

backend/drive/drive_test.go

@@ -1,4 +1,7 @@
 // Test Drive filesystem interface
+
+// +build go1.9
+
 package drive
 
 import (

backend/drive/drive_unsupported.go

@@ -0,0 +1,6 @@
+// Build for unsupported platforms to stop go complaining
+// about "no buildable Go source files "
+
+// +build !go1.9
+
+package drive

backend/drive/upload.go

@@ -8,6 +8,8 @@
 //
 // This contains code adapted from google.golang.org/api (C) the GO AUTHORS
 
+// +build go1.9
+
 package drive
 
 import (

backend/googlecloudstorage/googlecloudstorage.go

@@ -1,4 +1,7 @@
 // Package googlecloudstorage provides an interface to Google Cloud Storage
+
+// +build go1.9
+
 package googlecloudstorage
 
 /*

backend/googlecloudstorage/googlecloudstorage_test.go

@@ -1,4 +1,7 @@
 // Test GoogleCloudStorage filesystem interface
+
+// +build go1.9
+
 package googlecloudstorage_test
 
 import (

backend/googlecloudstorage/googlecloudstorage_unsupported.go

@@ -0,0 +1,6 @@
+// Build for unsupported platforms to stop go complaining
+// about "no buildable Go source files "
+
+// +build !go1.9
+
+package googlecloudstorage