Fixed travis build script.

Upgraded from xcode 9.2 to 9.4.
Use yarn instead of npm for windows and linux builds.
Unpin minor version of node in package.json.

.travis.yml

@@ -1,7 +1,7 @@
 matrix:
   include:
     - os: osx
-      osx_image: xcode9.2
+      osx_image: xcode9.4
       language: node_js
       node_js: "10"
       env:
@@ -27,6 +27,7 @@ before_install:
       mkdir -p /tmp/git-lfs && curl -L https://github.com/github/git-lfs/releases/download/v2.3.1/git-lfs-$([ "$TRAVIS_OS_NAME" == "linux" ] && echo "linux" || echo "darwin")-amd64-2.3.1.tar.gz | tar -xz -C /tmp/git-lfs --strip-components 1
       export PATH="/tmp/git-lfs:$PATH"
     fi
+
 before_script:
   - git lfs pull
 
@@ -39,10 +40,11 @@ script:
         -v ~/.cache/electron:/root/.cache/electron \
         -v ~/.cache/electron-builder:/root/.cache/electron-builder \
         electronuserland/builder:wine \
-        /bin/bash -c "npm install && npm run travislinux"
+        /bin/bash -c "yarn --link-duplicates --pure-lockfile && yarn travislinux"
     else
       npm run travisdarwin
     fi
+
 before_cache:
   - rm -rf $HOME/.cache/electron-builder/wine
 

package.json

@@ -15,7 +15,7 @@
     "dist:all": "npm run dist -- -wl --x64"
   },
   "engines": {
-    "node": "10.0.x"
+    "node": "10.x.x"
   },
   "repository": {
     "type": "git",