vendor: bump google/certificate-transparency-go to 1.0.20

This is to include the Windows + Go1.11 fix
(google/certificate-transparency-go#284).

Signed-off-by: Kir Kolyshkin <[email protected]>

Author: kolyshkin

vendor.conf

@@ -129,7 +129,7 @@ github.com/docker/swarmkit 68266392a176434d282760d2d6d0ab4c68edcae6
 github.com/gogo/protobuf v1.0.0
 github.com/cloudflare/cfssl 1.3.2
 github.com/fernet/fernet-go 1b2437bc582b3cfbb341ee5a29f8ef5b42912ff2
-github.com/google/certificate-transparency-go 5ab67e519c93568ac3ee50fd6772a5bcf8aa460d
+github.com/google/certificate-transparency-go v1.0.20
 golang.org/x/crypto 1a580b3eff7814fc9b40602fd35256c63b50f491
 golang.org/x/time a4bde12657593d5e90d0533a3e4fd95e635124cb
 github.com/hashicorp/go-memdb cb9a474f84cc5e41b273b20c6927680b2a8776ad