fix(build): update cross-env to v5.2.0

The v5.2.0 includes (amongst other things) a fix for incorrectly reported
exit codes of child processes, which can prevent build build failures from
correctly halting a build, resulting in incomplete or invalid builds.

See kentcdodds/cross-env#150 (comment)

package.json

@@ -157,7 +157,7 @@
     "browserslist": "^2.11.0",
     "chalk": "^2.0.1",
     "concurrently": "^3.5.1",
-    "cross-env": "^5.0.0",
+    "cross-env": "^5.2.0",
     "cross-spawn": "^6.0.5",
     "css-loader": "^0.28.11",
     "electron-builder": "20.15.0",

yarn.lock

@@ -2973,11 +2973,11 @@ create-react-class@^15.6.0:
     loose-envify "^1.3.1"
     object-assign "^4.1.1"
 
-cross-env@^5.0.0:
-  version "5.0.1"
-  resolved "https://registry.yarnpkg.com/cross-env/-/cross-env-5.0.1.tgz#ff4e72ea43b47da2486b43a7f2043b2609e44913"
+cross-env@^5.2.0:
+  version "5.2.0"
+  resolved "https://registry.yarnpkg.com/cross-env/-/cross-env-5.2.0.tgz#6ecd4c015d5773e614039ee529076669b9d126f2"
   dependencies:
-    cross-spawn "^5.1.0"
+    cross-spawn "^6.0.5"
     is-windows "^1.0.0"
 
 cross-spawn@^3.0.0: