Update .travis.yml

Author: luc-github

.travis.yml

@@ -1,20 +1,24 @@
-sudo: false
+dist: xenial
+sudo: require
 
 language: python
- python:
-     - "2.7"
+
+python:
+  - "3.7"
 
 os:
   - linux
 
 cache:
-     directories:
-         - "~/.platformio"
+  pip: true
+  directories:
+  - "~/.platformio"
+  
 install:
-     - pip install -U platformio
-     - platformio update
+  - pip install -U platformio
 
 before_script:
+  - platformio update
   - wget http://downloads.arduino.cc/arduino-1.8.5-linux64.tar.xz
   - tar xf arduino-1.8.5-linux64.tar.xz
   - mv arduino-1.8.5 $HOME/arduino_ide