Updated yarn to 0.18.1

Hopefully this will fix the 'module not found' error that was cropping
up with node-sass, which was causing deploy failures on CI.

pr #2188

Dockerfile-node

@@ -5,7 +5,7 @@ RUN apt-get update && apt-get install libelf1
 
 RUN adduser --disabled-password --gecos "" mitodl
 
-RUN npm install -g yarn@0.17.10
+RUN npm install -g yarn@0.18.1
 
 RUN mkdir -p /home/mitodl/.cache/yarn
 

package.json

@@ -112,7 +112,7 @@
   },
   "engines": {
     "node": "6.2.0",
-    "yarn": "0.17.10"
+    "yarn": "0.18.1"
   },
   "scripts": {
     "postinstall": "./webpack_if_prod.sh",

travis/Dockerfile-travis-watch-build

@@ -8,7 +8,7 @@ LABEL maintainer "ODL DevOps <[email protected]>"
 
 RUN apt-get update && apt-get install libelf1
 
-RUN npm install -g yarn@0.17.10
+RUN npm install -g yarn@0.18.1
 
 RUN mkdir /src