use dev version of platformio (specifying scm for platform is not supported in latest stable release)

.travis.yml

@@ -10,10 +10,8 @@ env:
 before_install:
   - nvm install $NODE_VERSION
 install:
-- pip install -U platformio
+- pip install -U https://github.com/platformio/platformio-core/archive/develop.zip
 - platformio lib install
-- platformio upgrade && platformio update
-- platformio platform install 'https://github.com/platformio/platform-espressif8266.git#feature/stage'
 - cd web && npm install && cd ..
 script:
 - platformio run -v