Update mender-convert to use Mender beta release v2.1.0b1

Changelog: Title

Signed-off-by: Lluis Campos <lluis.campos@northern.tech>

Dockerfile

@@ -1,6 +1,6 @@
 FROM ubuntu:18.04
 
-ARG MENDER_ARTIFACT_VERSION=3.0.1
+ARG MENDER_ARTIFACT_VERSION=3.1.0b1
 ARG GOLANG_VERSION=1.11.2
 
 RUN apt-get update && apt-get install -y \

device-image-shell/Dockerfile

@@ -1,6 +1,6 @@
 FROM ubuntu:18.04
 
-ARG MENDER_ARTIFACT_VERSION=3.0.1
+ARG MENDER_ARTIFACT_VERSION=3.1.0b1
 
 RUN apt-get update && apt-get install -y \
     wget \

docker-build

@@ -6,7 +6,7 @@ if [ -z "$IMAGE_NAME" ]; then
     IMAGE_NAME=mender-convert
 fi
 
-MENDER_CLIENT_VERSION="2.0.1"
+MENDER_CLIENT_VERSION="2.1.0b1"
 
 DOCKER_ARGS="--build-arg mender_client_version=${MENDER_CLIENT_VERSION}"