Add Greenkeeper dependency

package.json

@@ -57,6 +57,7 @@
     "coveralls": "^3.0.1",
     "enzyme": "^3.3.0",
     "enzyme-adapter-react-16": "^1.1.1",
+    "greenkeeper-lockfile": "^1.15.1",
     "gzip-size": "^4.1.0",
     "jest": "^23.1.0",
     "jest-enzyme": "^6.0.1",

yarn.lock

@@ -1760,6 +1760,14 @@ graceful-fs@^4.1.11, graceful-fs@^4.1.2, graceful-fs@^4.1.4:
   version "4.1.11"
   resolved "https://registry.yarnpkg.com/graceful-fs/-/graceful-fs-4.1.11.tgz#0e8bdfe4d1ddb8854d64e04ea7c00e2a026e5658"
 
+greenkeeper-lockfile@^1.15.1:
+  version "1.15.1"
+  resolved "https://registry.yarnpkg.com/greenkeeper-lockfile/-/greenkeeper-lockfile-1.15.1.tgz#289728213f8d4731596fdb011e3c6eb24ee96a73"
+  dependencies:
+    lodash "^4.17.4"
+    require-relative "^0.8.7"
+    semver "^5.3.0"
+
 growly@^1.3.0:
   version "1.3.0"
   resolved "https://registry.yarnpkg.com/growly/-/growly-1.3.0.tgz#f10748cbe76af964b7c96c93c6bcc28af120c081"
@@ -3654,7 +3662,7 @@ require-main-filename@^1.0.1:
   version "1.0.1"
   resolved "https://registry.yarnpkg.com/require-main-filename/-/require-main-filename-1.0.1.tgz#97f717b69d48784f5f526a6c5aa8ffdda055a4d1"
 
-require-relative@0.8.7:
+require-relative@0.8.7, require-relative@^0.8.7:
   version "0.8.7"
   resolved "https://registry.yarnpkg.com/require-relative/-/require-relative-0.8.7.tgz#7999539fc9e047a37928fa196f8e1563dabd36de"