Update module cenkalti/backoff to v4

Author: renovate-bot

go.sum

@@ -29,6 +29,7 @@ github.com/beorn7/perks v1.0.0/go.mod h1:KWe93zE9D1o94FZ5RNwFwVgaQK1VOXiVxmqh+Ce
 github.com/bombsimon/wsl v1.2.1/go.mod h1:43lEF/i0kpXbLCeDXL9LMT8c92HyBywXb0AsgMHYngM=
 github.com/cenkalti/backoff v2.2.0+incompatible h1:8qVbEY6GLhoLlLi1Ac2ZkVhedNwlhQXc39qivKp9+GI=
 github.com/cenkalti/backoff v2.2.0+incompatible/go.mod h1:b6Nc7NRH5C4aCISLry0tLnTjcuTEvoiqcWDdsU0sOGM=
+github.com/cenkalti/backoff v2.2.1+incompatible h1:tNowT99t7UNflLxfYYSlKYsBpXdEet03Pg2g16Swow4=
 github.com/cenkalti/backoff v2.2.1+incompatible/go.mod h1:90ReRw6GdpyfrHakVjL/QHaoyV4aDUVVkXQJJJ3NXXM=
 github.com/cenkalti/backoff/v4 v4.0.0/go.mod h1:eEew/i+1Q6OrCDZh3WiXYv3+nJwBASZ8Bog/87DQnVg=
 github.com/certifi/gocertifi v0.0.0-20190506164543-d2eda7129713 h1:UNOqI3EKhvbqV8f1Vm3NIwkrhq388sGCeAH2Op7w0rc=
@@ -231,6 +232,7 @@ github.com/kr/text v0.1.0 h1:45sCR5RtlFHMR4UwH9sdQ5TC8v0qDQCHnXt+kaKSTVE=
 github.com/kr/text v0.1.0/go.mod h1:4Jbv+DJW3UT/LiOwJeYQe1efqtUx/iVham/4vfdArNI=
 github.com/levigross/grequests v0.0.0-20190130132859-37c80f76a0da h1:ixpx9UaTDElZrjbd9GeOVG4Deut0FFumoeel7PvVNm4=
 github.com/levigross/grequests v0.0.0-20190130132859-37c80f76a0da/go.mod h1:uCZIhROSrVmuF/BPYFPwDeiiQ6juSLp0kikFoEcNcEs=
+github.com/levigross/grequests v0.0.0-20190908174114-253788527a1a h1:DGFy/362j92vQRE3ThU1yqg9TuJS8YJOSbQuB7BP9cA=
 github.com/levigross/grequests v0.0.0-20190908174114-253788527a1a/go.mod h1:jVntzcUU+2BtVohZBQmSHWUmh8B55LCNfPhcNCIvvIg=
 github.com/lib/pq v1.1.1 h1:sJZmqHoEaY7f+NPP8pgLB/WxulyR3fewgCM2qaSlBb4=
 github.com/lib/pq v1.1.1/go.mod h1:5WUZQaWbwv1U+lTReE5YruASi9Al49XbQIvNi/34Woo=