Update Yarn to the latest version

This should fix the warnings, see: #883 (comment)
date-fns · Jun 7, 2019 · a697c5f · a697c5f
1 parent 6872aba
commit a697c5f
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diff --git a/.travis.yml b/.travis.yml
@@ -17,6 +17,7 @@ before_install:
   - export CHROME_BIN=chromium-browser
   - export DISPLAY=:99.0
   - sh -e /etc/init.d/xvfb start
+  - npm install -g yarn
 
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