master branch of embark-docker should spec 'latest' (released) ver of embark

Dockerfile

@@ -5,10 +5,10 @@ ARG __LC_ALL=en_US.${__CODESET}
 ARG BASHIT_VERSION=10-aug-2018
 ARG BUILDER_BASE_IMAGE=buildpack-deps
 ARG BUILDER_BASE_TAG=stretch
-ARG EMBARK_VERSION=3.1.5
 ARG GETH_VERSION=1.8.11-dea1ce05
 ARG IPFS_VERSION=0.4.15
 ARG MICRO_VERSION=1.4.0
+ARG EMBARK_VERSION=latest
 ARG GANACHE_VERSION=6.1.0
 ARG NODE_VERSION=8.11.3
 ARG NODEENV_VERSION=1.3.2