[ci] use Node.js 10 for compatibility with Gulp v3

It seems Node.js 12 (which is now 'latest') does not work with Gulp v3. In the future, we will
either have to migrate to Gulp v4, or remove Gulp from the build system.

See gulpjs/gulp#2324

Author: darrachequesne

.travis.yml

@@ -7,7 +7,7 @@ git:
   depth: 1
 matrix:
   include:
-    - node_js: node
+    - node_js: '10'
       env: BROWSERS=1
 cache:
   directories: