Update cached @PlatformIO packages (#9124)

11 line caches PIO packages (toolchains, libs) which is very good! This PR will force PIO to find updates for installed libs and packages.

.travis.yml

   - pip install -U platformio
 
 before_script:
+  # Update PlatformIO packages
+  - platformio update
   #
   # Change current working directory to the build dir
   - cd ${TRAVIS_BUILD_DIR}