Bump to artful

Zesty fell out of support on Jan 13, and the packages have
started disappearing from http://archive.ubuntu.com/ubuntu/ and
other mirrors as of today. This will totally screw up our
build processes whenever it hits the mirror we use.

repo2docker/buildpacks/base.py

@@ -10,7 +10,7 @@ import re
 import docker
 
 TEMPLATE = r"""
-FROM buildpack-deps:zesty
+FROM buildpack-deps:artful
 
 # Set up locales properly
 RUN apt-get update && \
@@ -151,7 +151,6 @@ class BuildPack(LoggingConfigurable):
             # FIXME: Use npm from nodesource!
             # Everything seems to depend on npm these days, unfortunately.
             "npm",
-            "nodejs-legacy"
         },
         help="""
         Base set of apt packages that are installed for all images.