go.mod: update proto package

The previous version breaks for 1.13.

Change-Id: I8c9c6b9244ec9a811cf980c61ac2c582929069bf
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/3021
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

go.mod

@@ -3,13 +3,14 @@ module cuelang.org/go
 require (
 	github.com/cockroachdb/apd v1.1.0 // indirect
 	github.com/cockroachdb/apd/v2 v2.0.1
-	github.com/emicklei/proto v1.6.11
+	github.com/emicklei/proto v1.6.15
 	github.com/ghodss/yaml v1.0.0
 	github.com/google/go-cmp v0.2.0
 	github.com/kr/pretty v0.1.0
 	github.com/kylelemons/godebug v1.1.0
 	github.com/lib/pq v1.0.0 // indirect
 	github.com/mpvl/unique v0.0.0-20150818121801-cbe035fff7de
+	github.com/pkg/errors v0.8.1 // indirect
 	github.com/retr0h/go-gilt v0.0.0-20190206215556-f73826b37af2
 	github.com/spf13/cobra v0.0.3
 	github.com/spf13/pflag v1.0.3

go.sum

@@ -5,6 +5,10 @@ github.com/cockroachdb/apd/v2 v2.0.1/go.mod h1:DDxRlzC2lo3/vSlmSoS7JkqbbrARPuFOG
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/emicklei/proto v1.6.11 h1:KZHE0+iwVLth2D/K8jat9rs70K6TFWyol8ihrOdrbM0=
 github.com/emicklei/proto v1.6.11/go.mod h1:rn1FgRS/FANiZdD2djyH7TMA9jdRDcYQ9IEN9yvjX0A=
+github.com/emicklei/proto v1.6.13 h1:8iuAuKbFmFhkmstObb0EV/Hrn9W+x6EuV1y5Da8Ye9E=
+github.com/emicklei/proto v1.6.13/go.mod h1:rn1FgRS/FANiZdD2djyH7TMA9jdRDcYQ9IEN9yvjX0A=
+github.com/emicklei/proto v1.6.15 h1:XbpwxmuOPrdES97FrSfpyy67SSCV/wBIKXqgJzh6hNw=
+github.com/emicklei/proto v1.6.15/go.mod h1:rn1FgRS/FANiZdD2djyH7TMA9jdRDcYQ9IEN9yvjX0A=
 github.com/ghodss/yaml v1.0.0 h1:wQHKEahhL6wmXdzwWG11gIVCkOv05bNOh+Rxn0yngAk=
 github.com/ghodss/yaml v1.0.0/go.mod h1:4dBDuWmgqj2HViK6kFavaiC9ZROes6MMH2rRYeMEF04=
 github.com/google/go-cmp v0.2.0 h1:+dTQ8DZQJz0Mb/HjFlkptS1FeQ4cWSnN941F8aEG4SQ=