chore(package): update lockfile

https://npm.im/greenkeeper-lockfile

Author: greenkeeperio-bot

yarn.lock

@@ -3774,9 +3774,9 @@ js-tokens@^3.0.0, js-tokens@^3.0.2:
   version "3.0.2"
   resolved "https://registry.yarnpkg.com/js-tokens/-/js-tokens-3.0.2.tgz#9866df395102130e38f7f996bceb65443209c25b"
 
-js-yaml@3.11.0:
-  version "3.11.0"
-  resolved "https://registry.yarnpkg.com/js-yaml/-/js-yaml-3.11.0.tgz#597c1a8bd57152f26d622ce4117851a51f5ebaef"
+js-yaml@3.12.0:
+  version "3.12.0"
+  resolved "https://registry.yarnpkg.com/js-yaml/-/js-yaml-3.12.0.tgz#eaed656ec8344f10f527c6bfa1b6e2244de167d1"
   dependencies:
     argparse "^1.0.7"
     esprima "^4.0.0"