Use NPM 6.0.1

Dockerfile

@@ -19,7 +19,7 @@ ENV NODE_VERSION ${NODE_VERSION:-v8.11.1}
 ENV METEOR_RELEASE ${METEOR_RELEASE:-1.6.0.1}
 ENV USE_EDGE ${USE_EDGE:-false}
 ENV METEOR_EDGE ${METEOR_EDGE:-1.5-beta.17}
-ENV NPM_VERSION ${NPM_VERSION:-5.5.1}
+ENV NPM_VERSION ${NPM_VERSION:-6.0.1}
 ENV FIBERS_VERSION ${FIBERS_VERSION:-2.0.0}
 ENV ARCHITECTURE ${ARCHITECTURE:-linux-x64}
 ENV SRC_PATH ${SRC_PATH:-./}

snapcraft.yaml

@@ -83,7 +83,7 @@ parts:
         plugin: nodejs
         node-engine: 8.11.1
         node-packages:
-            - npm@5.5.1
+            - npm@6.0.1
             - node-gyp
             - node-pre-gyp
             - fibers@2.0.0