have travis check for uncommited yarn.lock changes

.travis.yml

@@ -5,4 +5,6 @@ before_install:
   - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.5.1
   - export PATH="$HOME/.yarn/bin:$PATH"
 install: yarn 
-script: yarn lint
+script:
+- yarn lint
+- ./scripts/check-yarn-lock.sh

scripts/check-yarn-lock.sh

@@ -0,0 +1,10 @@
+#!/usr/bin/env bash
+
+if [ `git diff yarn.lock | wc -l` -ne 0 ]; then
+  echo ""
+  tput setaf 1
+  echo "!! UNCOMMITED YARN.LOCK CHANGES !!"
+  tput sgr0
+  echo ""
+  exit 1
+fi