Fix setting environment var on windows

Use platform-specific commands to set NODE_ENV after yarn install but
before yarn travis-build.

// FREEBIE

.travis.yml

@@ -8,6 +8,7 @@ dist: trusty
 install:
   - yarn install
 script:
+  - export NODE_ENV=production
   - yarn test
   - yarn travis-build
 env:

appveyor.yml

@@ -14,4 +14,5 @@ test_script:
   - yarn test
 
 build_script:
+  - set NODE_ENV=production
   - yarn travis-build

package.json

@@ -35,7 +35,7 @@
     "test": "grunt test",
     "lint": "grunt jshint",
     "start": "electron .",
-    "travis-build": "npm run icon-gen && grunt && NODE_ENV=production build --em.environment=$NODE_ENV --config.mac.bundleVersion='$TRAVIS_BUILD_NUMBER'",
+    "travis-build": "npm run icon-gen && grunt && build --em.environment=$NODE_ENV --config.mac.bundleVersion='$TRAVIS_BUILD_NUMBER'",
     "dist": "npm run icon-gen && grunt && build --em.environment=$NODE_ENV",
     "pack": "npm run dist -- --dir",
     "pack-staging": "NODE_ENV=staging npm run pack",