Update packages

Webpack is pinned because versions higher than this cause the viewer not
to work (see https://github.com/mozilla/pdf.js/pull/10170#issuecomment-431697032).

Node.js is pinned for Travis CI because version 11 requires that we
update to Gulp 4.

.travis.yml

@@ -1,6 +1,6 @@
 language: node_js
 node_js:
-  - node
+  - "10"
 cache:
   directories:
     - node_modules

package.json

@@ -11,7 +11,7 @@
     "babel-preset-env": "^1.7.0",
     "core-js": "^2.5.7",
     "escodegen": "^1.11.0",
-    "eslint": "^5.6.1",
+    "eslint": "^5.7.0",
     "eslint-plugin-mozilla": "^0.14.0",
     "eslint-plugin-no-unsanitized": "^3.0.2",
     "eslint-plugin-unicorn": "^4.0.3",
@@ -37,7 +37,7 @@
     "uglify-es": "^3.3.9",
     "vinyl": "^2.2.0",
     "vinyl-fs": "^3.0.3",
-    "webpack": "^4.20.2",
+    "webpack": "4.21.0",
     "webpack-stream": "^4.0.3",
     "wintersmith": "^2.4.1",
     "yargs": "^11.1.0"