Update mongo-express to 0.31.0

[knickers]

This mongo-express update includes the following:

• Use the latest node 5 version
• Adds a gridFSEnabled option in the config file
• Fix a DBRef field broken error
• Fix a variable name typo

[knickers]

Build test of #2080; 5c054b2 (mongo-express):

$ bashbrew build mongo-express:0.31.0
warning: insecure protocol git:// detected: git://github.com/mongo-express/mongo-express-docker
Building bashbrew/cache:fc94f30d4a4245dd24bdea357ac3c8c172b96677c879db319a2a513f1ad10554 (mongo-express:0.31.0)
Tagging mongo-express:0.31.0
Tagging mongo-express:0.31
Tagging mongo-express:latest

$ test/run.sh mongo-express:0.31.0
testing mongo-express:0.31.0
    'utc' [1/4]...passed
    'cve-2014--shellshock' [2/4]...passed
    'no-hard-coded-passwords' [3/4]...passed
    'override-cmd' [4/4]...passed

[tianon]

warning: insecure protocol git:// detected: git://github.com/mongo-express/mongo-express-docker
diff --git a/mongo-express_latest/Dockerfile b/mongo-express_latest/Dockerfile
index 3a7688e..62782e9 100644
--- a/mongo-express_latest/Dockerfile
+++ b/mongo-express_latest/Dockerfile
@@ -1,4 +1,4 @@
-FROM node:5.6-slim
+FROM node:5-slim

 # grab tini for signal processing and zombie killing
 ENV TINI_VERSION 0.9.0
@@ -26,7 +26,7 @@ ENV ME_CONFIG_EDITORTHEME="default" \
     ME_CONFIG_BASICAUTH_PASSWORD="" \
     VCAP_APP_HOST="0.0.0.0"

-ENV MONGO_EXPRESS 0.30.59
+ENV MONGO_EXPRESS 0.31.0

 RUN npm install mongo-express@$MONGO_EXPRESS

[tianon]

LGTM