Travis: Use platformio 'master' for stable testing (#13902)

.travis.yml

@@ -45,7 +45,7 @@ before_install:
 
 install:
   #- pip install -U platformio
-  - pip install -U https://github.com/platformio/platformio-core/archive/develop.zip
+  - pip install -U https://github.com/platformio/platformio-core/archive/master.zip
 
 before_script:
   # Update PlatformIO packages