tests: drop travisci usage

.travis.yml

@@ -1,19 +0,0 @@
-dist: bionic
-language: bash
-env:
-  - DOKKU_VERSION=master
-  - DOKKU_VERSION=v0.19.0
-install: make setup
-before_script: sudo sysctl -w vm.max_map_count=262144
-script:
-  # Check that README etc. is up to date
-  - >
-    make generate;
-    if [[ $(git diff) ]]; then
-      echo "Please run `make generate`";
-      git status --short;
-      git diff;
-      exit 1;
-    fi
-  - make test
-after_failure: make report