Simplify docker version sed expression (close #282)

weave

@@ -396,7 +396,7 @@ if ! ip netns list >/dev/null 2>&1 ; then
     exit 1
 fi
 
-if ! DOCKER_VERSION=$(docker -v | sed -n 's|^Docker version \([0-9]\+\.[0-9]\+.[0-9]\+\),.*|\1|p') || [ -z "$DOCKER_VERSION" ] ; then
+if ! DOCKER_VERSION=$(docker -v | sed -n 's|^Docker version \([0-9]\+\.[0-9]\+.[0-9]\+\).*|\1|p') || [ -z "$DOCKER_VERSION" ] ; then
     echo "ERROR: Unable to parse docker version" >&2
     exit 1
 fi