Depend on latest newtmgr

This fixes #8.
diff --git a/go.mod b/go.mod
index ffcf542..571f919 100644
--- a/go.mod
+++ b/go.mod
@@ -4,5 +4,5 @@
 
 require (
 	mynewt.apache.org/newt v0.0.0-20190723172213-9f7829a76fef
-	mynewt.apache.org/newtmgr v0.0.0-20190801165443-4ba7e39aca9f
+	mynewt.apache.org/newtmgr v0.0.0-20190808223606-13b828d6f85f
 )
diff --git a/go.sum b/go.sum
index 9168603..306121a 100644
--- a/go.sum
+++ b/go.sum
@@ -1,6 +1,7 @@
 github.com/BurntSushi/toml v0.3.1/go.mod h1:xHWCNGjB5oqiDr8zfno3MHue2Ht5sIBksp03qcyfWMU=
 github.com/NickBall/go-aes-key-wrap v0.0.0-20170929221519-1c3aa3e4dfc5/go.mod h1:w5D10RxC0NmPYxmQ438CC1S07zaC1zpvuNW7s5sUk2Q=
 github.com/abiosoft/ishell v2.0.0+incompatible/go.mod h1:HQR9AqF2R3P4XXpMpI0NAzgHf/aS6+zVXRj14cVk9qg=
+github.com/abiosoft/readline v0.0.0-20180607040430-155bce2042db h1:CjPUSXOiYptLbTdr1RceuZgSFDQ7U15ITERUGrUORx8=
 github.com/abiosoft/readline v0.0.0-20180607040430-155bce2042db/go.mod h1:rB3B4rKii8V21ydCbIzH5hZiCQE7f5E9SzUb/ZZx530=
 github.com/apache/mynewt-artifact v0.0.3/go.mod h1:vFUd47t74KPQMzSBhQ2qp5Hc7D29OU/Tl3xHtFwN3k8=
 github.com/armon/consul-api v0.0.0-20180202201655-eb2c6b5be1b6/go.mod h1:grANhF5doyWs3UAsr3K4I6qtAmlQcZDesFNEHPZAzj8=
@@ -50,6 +51,7 @@
 github.com/pkg/errors v0.8.1 h1:iURUrRGxPUNPdy5/HRSm+Yj6okJ6UtLINN0Q9M4+h3I=
 github.com/pkg/errors v0.8.1/go.mod h1:bwawxfHBFNV+L2hUp1rHADufV3IMtnDRdf1r5NINEl0=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/raff/goble v0.0.0-20170814221208-b12b34f940c4 h1:wa4KKsyv+e7TmpzAfp+TWXG+3s/ZWaQ4QsVqMU5oUrk=
 github.com/raff/goble v0.0.0-20170814221208-b12b34f940c4/go.mod h1:CxaUhijgLFX0AROtH5mluSY71VqpjQBw9JXE2UKZmc4=
 github.com/runtimeco/go-coap v0.0.0-20180109101419-47c23e2a63ea h1:zTOcylKcdobkyW4gySyvvHkAGnOP0QNcGaO0ZQJFgxQ=
 github.com/runtimeco/go-coap v0.0.0-20180109101419-47c23e2a63ea/go.mod h1:x/rLuxedbWFDN497ZirUaDklkWz6JhZLjGdgurMysxM=
@@ -100,6 +102,8 @@
 golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
 gopkg.in/abiosoft/ishell.v1 v1.0.0-20171224170712-50251d04cb42 h1:SC3lEzz9Nt8Ur/Xn7AUS8qtjtnyDUOG3pB7hJ8JT1qM=
 gopkg.in/abiosoft/ishell.v1 v1.0.0-20171224170712-50251d04cb42/go.mod h1:3LjPMJqZQyIeenUuJm+3oamADXoWVncBdPuAEY3SNZ4=
+gopkg.in/abiosoft/ishell.v2 v2.0.0 h1:/J5yh3nWYSSGFjALcitTI9CLE0Tu27vBYHX0srotqOc=
+gopkg.in/abiosoft/ishell.v2 v2.0.0/go.mod h1:sFp+cGtH6o4s1FtpVPTMcHq2yue+c4DGOVohJCPUzwY=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/cheggaaa/pb.v1 v1.0.28/go.mod h1:V/YB90LKu/1FcN3WVnfiiE5oMCibMjukxqG/qStrOgw=
 gopkg.in/yaml.v2 v2.2.2/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
@@ -108,3 +112,5 @@
 mynewt.apache.org/newt v0.0.0-20190723172213-9f7829a76fef/go.mod h1:oOnor4yY/stqy8RmNmk5UAE9zssKixi68pl805wtCVo=
 mynewt.apache.org/newtmgr v0.0.0-20190801165443-4ba7e39aca9f h1:6su567b13mgaBMakTo92GFboYjT7KGoJdMYBszd9UEI=
 mynewt.apache.org/newtmgr v0.0.0-20190801165443-4ba7e39aca9f/go.mod h1:AEQCGhOKwH5FxIzQCNwvdRP3kd7Uuje9iwVElZ06m34=
+mynewt.apache.org/newtmgr v0.0.0-20190808223606-13b828d6f85f h1:0YrF0m0kNzJKyc3zIawrFOW47mse3MoNcD3pZN3YtGA=
+mynewt.apache.org/newtmgr v0.0.0-20190808223606-13b828d6f85f/go.mod h1:Jg/LsXcv9PVzekouYdz78eZnTFN+jdCqhLTJTo24f9c=