maint: electron builder update

Also teste app build on Mac 10.13.6 (xcode 10) which is required now for code signing (https://github.com/electron-userland/electron-builder/releases/tag/v21.0.10). Per Julie, works fine with 10.12.5 OSX, which is good. Will confirm signing is working correctly.

cleanup: bump proper versions

.travis.yml

@@ -2,7 +2,7 @@ matrix:
   include:
     - os: osx
       env: TARGET=mac
-      osx_image: xcode9.2
+      osx_image: xcode10
       language: node_js
       node_js: '10'
     - os: linux

package.json

@@ -44,7 +44,7 @@
     "@reach/tooltip": "^0.2.1",
     "electron-dl": "^1.11.0",
     "electron-log": "^2.2.12",
-    "electron-updater": "^4.0.6",
+    "electron-updater": "^4.1.2",
     "express": "^4.16.4",
     "keytar": "^4.4.1",
     "tiny-relative-date": "^1.3.0"
@@ -95,7 +95,7 @@
     "devtron": "^1.4.0",
     "dom-scroll-into-view": "^1.2.1",
     "electron": "4.1.0",
-    "electron-builder": "^20.41.0",
+    "electron-builder": "^21.2.0",
     "electron-devtools-installer": "^2.2.4",
     "electron-is-dev": "^0.3.0",
     "electron-publisher-s3": "^20.8.1",