build(webpack): Fix incremental webpack builds (upgrade to node@8.15.1)

Upgrades to node@8.15.1 and yarn@1.13.0 to be consistent with rest of sentry

Perf regression in node 8.10.0 - see nodejs/node#19769
See getsentry/sentry#12408

.travis.yml

@@ -24,8 +24,8 @@ env:
   global:
     - PIP_DISABLE_PIP_VERSION_CHECK=on
     - SENTRY_SKIP_BACKEND_VALIDATION=1
-    - TRAVIS_NODE_VERSION=8.10.0
-    - YARN_VERSION="1.3.2"
+    - TRAVIS_NODE_VERSION=8.15.1
+    - YARN_VERSION="1.13.0"
 install:
   - 'export PATH=$PATH:~/.bin'
   - rvm install 2.2.5