glide: fix DeleteUnused flag

The `--delete` flag in the `glide update` subcommand actually didn't do
anything.

This fixes it.

glide.go

diff --git a/glide.go b/glide.go
index 149c060..0e1e355 100644
--- a/glide.go
+++ b/glide.go
@@ -446,7 +446,7 @@
 				installer.UseCacheGopath = c.Bool("cache-gopath")
 				installer.UpdateVendored = c.Bool("update-vendored")
 				installer.Home = c.GlobalString("home")
-				installer.DeleteUnused = c.Bool("deleteOptIn")
+				installer.DeleteUnused = c.Bool("delete")
 
 				action.Install(installer, c.Bool("strip-vcs"), c.Bool("strip-vendor"))
 			},
@@ -559,7 +559,7 @@
 				installer.UpdateVendored = c.Bool("update-vendored")
 				installer.ResolveAllFiles = c.Bool("all-dependencies")
 				installer.Home = c.GlobalString("home")
-				installer.DeleteUnused = c.Bool("deleteOptIn")
+				installer.DeleteUnused = c.Bool("delete")
 
 				action.Update(installer, c.Bool("no-recursive"), c.Bool("strip-vcs"), c.Bool("strip-vendor"))
 			},