build(TravisCI): execute npm run build command

instead of having another explicit build command

.travis.yml

@@ -8,7 +8,7 @@ install:
   - npm install
 
 script:
-  - "./node_modules/.bin/pkg --targets node8-linux-armv7 --no-bytecode --options max-old-space-size=72 --public-packages=exif-parser,omggif,trim,prettycron,mqtt ."
+  - npm run build
 
 deploy:
   provider: releases