Upgrade cross-env to v5

package.json

@@ -31,7 +31,7 @@
     "babel-preset-react": "6.24.1",
     "babel-preset-stage-0": "6.24.1",
     "classnames": "2.2.5",
-    "cross-env": "^4.0.0",
+    "cross-env": "^5.1.1",
     "css-mqpacker": "^6.0.1",
     "front-matter": "2.3.0",
     "fs-extra": "4.0.2",

yarn.lock

@@ -1320,9 +1320,9 @@ create-hmac@^1.1.0, create-hmac@^1.1.2:
     create-hash "^1.1.0"
     inherits "^2.0.1"
 
-cross-env@^4.0.0:
-  version "4.0.0"
-  resolved "https://registry.yarnpkg.com/cross-env/-/cross-env-4.0.0.tgz#16083862d08275a4628b0b243b121bedaa55dd80"
+cross-env@^5.1.1:
+  version "5.1.1"
+  resolved "https://registry.yarnpkg.com/cross-env/-/cross-env-5.1.1.tgz#b6d8ab97f304c0f71dae7277b75fe424c08dfa74"
   dependencies:
     cross-spawn "^5.1.0"
     is-windows "^1.0.0"