Remove fix for old travis cache problem

.travis.yml

@@ -27,11 +27,6 @@ before_install:
   - if [ ! -d "$HOME/$ARDUINO_IDE" ] ; then { echo "fetch" && wget http://downloads.arduino.cc/arduino-${IDE_VERSION}-linux64.tar.xz && echo "untar" && tar xf arduino-${IDE_VERSION}-linux64.tar.xz && echo "move" && mv -f arduino-${IDE_VERSION} $HOME/${ARDUINO_IDE} ; } || { echo "IDE install failed"; exit 1; } ; else echo "IDE already installed" ; fi
   - ln -sf $PWD $HOME/${ARDUINO_IDE}/libraries/Test_Library
   - export PATH="$HOME/${ARDUINO_IDE}:$PATH"
-  # clean up for earlier bugs. This is transitional. The libraries were formerly (incorrectly) put at
-  # $ARDUINO_IDE/libraries -- this could result in them being cached. We don't want them to be cached.
-  # No need to add new libraries to this lise, only the first two were ever in the wrong place.
-  # We can remove this after 2018-11-01.
-  - rm -rf "$HOME/${ARDUINO_IDE}/libraries/Adafruit_Sensor" "$HOME/${ARDUINO_IDE}/libraries/DHT-sensor-library" "$HOME/${ARDUINO_IDE}/libraries/Time"
   #
   # functions to generate the board settings for SAMD, STM32L0, ...
   # keep args for these aligned for any common options. $1 is always board name, $2 is region.