go.mod: Update to mynewt-artifact 0.0.19

This adds some missing fields to the `imgmod image show` output.
diff --git a/go.mod b/go.mod
index a463cca..66114d1 100644
--- a/go.mod
+++ b/go.mod
@@ -1,7 +1,7 @@
 module mynewt.apache.org/imgmod
 
 require (
-	github.com/apache/mynewt-artifact v0.0.18
+	github.com/apache/mynewt-artifact v0.0.19
 	github.com/otiai10/copy v1.0.1
 	github.com/pkg/errors v0.8.1
 	github.com/sirupsen/logrus v1.4.2
diff --git a/go.sum b/go.sum
index 7d5fabe..a46f797 100644
--- a/go.sum
+++ b/go.sum
@@ -13,6 +13,8 @@
 github.com/apache/mynewt-artifact v0.0.17/go.mod h1:vFUd47t74KPQMzSBhQ2qp5Hc7D29OU/Tl3xHtFwN3k8=
 github.com/apache/mynewt-artifact v0.0.18 h1:E5em2q/hbkDU0/xSGJozWdKSo42XJcZVmSLj20G1KXQ=
 github.com/apache/mynewt-artifact v0.0.18/go.mod h1:vFUd47t74KPQMzSBhQ2qp5Hc7D29OU/Tl3xHtFwN3k8=
+github.com/apache/mynewt-artifact v0.0.19 h1:g/LjJvl0amG38INCEf1Gl/i0Z8wJl0dgN2elCcL0hIg=
+github.com/apache/mynewt-artifact v0.0.19/go.mod h1:vFUd47t74KPQMzSBhQ2qp5Hc7D29OU/Tl3xHtFwN3k8=
 github.com/armon/consul-api v0.0.0-20180202201655-eb2c6b5be1b6/go.mod h1:grANhF5doyWs3UAsr3K4I6qtAmlQcZDesFNEHPZAzj8=
 github.com/coreos/etcd v3.3.10+incompatible/go.mod h1:uF7uidLiAD3TWHmW31ZFd/JWoc32PjwdhPthX9715RE=
 github.com/coreos/go-etcd v2.0.0+incompatible/go.mod h1:Jez6KQU2B/sWsbdaef3ED8NzMklzPG4d5KIOhIy30Tk=