CI: upgrade to Ubuntu 18.04

Latest pylint doesn't support Python 3.5 (latest available on 16.04)
anymore.

.travis.yml

@@ -6,10 +6,10 @@ services:
 language: c
 
 before_install:
-  - docker pull ubuntu:16.04
+  - docker pull ubuntu:18.04
 
 script:
-  - docker run -v "$(pwd):/var/razer" ubuntu:16.04 /bin/sh -c "cd /var/razer/ && ./scripts/ci/ci.sh"
+  - docker run -v "$(pwd):/var/razer" ubuntu:18.04 /bin/sh -c "cd /var/razer/ && ./scripts/ci/ci.sh"
 
 notifications:
     webhooks:

scripts/ci/install-check-deps.sh

@@ -6,7 +6,6 @@ apt-get -y install \
     python3-pip
 
 pip3 install autopep8
-pip3 install 'isort<5' # isort 5+ requires Python 3.6+
 pip3 install pylint
 
 # pylint errors with 1.8+ on pylib/tests/integration_tests/test_device_manager.py about the missing 'coverage' module