(ci) Always try to update pip2 to latest version

Author: frazar

.travis.yml

@@ -37,6 +37,7 @@ before_install:
   - uname --all
   - sudo apt-get update -qq && sudo apt-get -y install python-pip
   - python2 --version
+  - pip2 install --user --upgrade pip
   - pip2 --version
   - pip2 install --user --upgrade platformio
   - platformio update