Update .travis.yml

Drop deprecated 'sudo' key
Update kernel headers to 5.4

.travis.yml

@@ -15,10 +15,9 @@ os:
 compiler: gcc
 
 dist: bionic
-sudo: required
 
 before_install:
-   - export KVER="5.3.0-1009-gcp"
+   - export KVER="5.4.0-1028-gcp"
    - export KSRC="/usr/src/linux-headers-$KVER"
    - sudo sh -c "echo 'deb http://archive.ubuntu.com/ubuntu/ $(lsb_release -cs)-proposed restricted main multiverse universe' >> /etc/apt/sources.list.d/proposed-repositories.list"
    - sudo apt-get update