Update packr dependency

Update packr to 2.8.1 which fixes this issue:
https://github.com/gobuffalo/packr/issues/261 which I encountered while running
a test with `-race`

go.mod

@@ -12,7 +12,7 @@ require (
 	github.com/gin-contrib/cors v1.3.1
 	github.com/gin-gonic/gin v1.5.0
 	github.com/go-sql-driver/mysql v1.5.0 // indirect
-	github.com/gobuffalo/packr/v2 v2.8.0
+	github.com/gobuffalo/packr/v2 v2.8.1
 	github.com/hermeznetwork/tracerr v0.3.1-0.20201126162137-de9930d0cf29
 	github.com/iden3/go-iden3-crypto v0.0.6-0.20201221160344-58e589b6eb4c
 	github.com/iden3/go-merkletree v0.0.0-20201215142017-730707e5659a

go.sum

@@ -237,6 +237,8 @@ github.com/gobuffalo/packd v1.0.0/go.mod h1:6VTc4htmJRFB7u1m/4LeMTWjFoYrUiBkU9Fd
 github.com/gobuffalo/packr/v2 v2.7.1/go.mod h1:qYEvAazPaVxy7Y7KR0W8qYEE+RymX74kETFqjFoFlOc=
 github.com/gobuffalo/packr/v2 v2.8.0 h1:IULGd15bQL59ijXLxEvA5wlMxsmx/ZkQv9T282zNVIY=
 github.com/gobuffalo/packr/v2 v2.8.0/go.mod h1:PDk2k3vGevNE3SwVyVRgQCCXETC9SaONCNSXT1Q8M1g=
+github.com/gobuffalo/packr/v2 v2.8.1 h1:tkQpju6i3EtMXJ9uoF5GT6kB+LMTimDWD8Xvbz6zDVA=
+github.com/gobuffalo/packr/v2 v2.8.1/go.mod h1:c/PLlOuTU+p3SybaJATW3H6lX/iK7xEz5OeMf+NnJpg=
 github.com/godror/godror v0.13.3/go.mod h1:2ouUT4kdhUBk7TAkHWD4SN0CdI0pgEQbo8FVHhbSKWg=
 github.com/gofrs/flock v0.7.1/go.mod h1:F1TvTiK9OcQqauNUHlbJvyl9Qa1QvF/gOUDKA14jxHU=
 github.com/gogo/googleapis v1.1.0/go.mod h1:gf4bu3Q80BeJ6H1S1vYPm8/ELATdvryBaNFGgqEef3s=
@@ -392,6 +394,8 @@ github.com/karalabe/usb v0.0.0-20190919080040-51dc0efba356 h1:I/yrLt2WilKxlQKCM5
 github.com/karalabe/usb v0.0.0-20190919080040-51dc0efba356/go.mod h1:Od972xHfMJowv7NGVDiWVxk2zxnWgjLlJzE+F4F7AGU=
 github.com/karrick/godirwalk v1.15.3 h1:0a2pXOgtB16CqIqXTiT7+K9L73f74n/aNQUnH6Ortew=
 github.com/karrick/godirwalk v1.15.3/go.mod h1:j4mkqPuvaLI8mp1DroR3P6ad7cyYd4c1qeJ3RV7ULlk=
+github.com/karrick/godirwalk v1.15.8 h1:7+rWAZPn9zuRxaIqqT8Ohs2Q2Ac0msBqwRdxNCr2VVs=
+github.com/karrick/godirwalk v1.15.8/go.mod h1:j4mkqPuvaLI8mp1DroR3P6ad7cyYd4c1qeJ3RV7ULlk=
 github.com/kevinburke/ssh_config v0.0.0-20190725054713-01f96b0aa0cd/go.mod h1:CT57kijsi8u/K/BOFA39wgDQJ9CxiF4nAY/ojJ6r6mM=
 github.com/kisielk/errcheck v1.1.0/go.mod h1:EZBBE59ingxPouuu3KfxchcWSUPOHkagtvWXihfKN4Q=
 github.com/kisielk/errcheck v1.2.0/go.mod h1:/BMXB+zMLi60iA8Vv6Ksmxu/1UDYcXs4uQLJ+jE2L00=