remove .travis.yml and travis badge from README.md (#1013)

Thanks to travis.org for the CI support for many years.

.travis.yml

-language: python
-sudo: required
-
-matrix:
-  include:
-    - dist: focal
-      python: 3.6
-      env: dist="24.04 LTS focal"
-
-before_install:
-- sudo apt-get update -qq
-- sudo apt-get install -qq vlan pyflakes
-- PYTHON=`which python` util/install.sh -n
-
-install:
-- make codecheck
-- pip install pexpect || pip3 install pexpect
-- util/install.sh -nfvw
-
-script:
-- alias sudo="sudo env PATH=$PATH"
-- export PYTHON=`which python`
-- echo 'px import sys; print(sys.version_info)' | sudo $PYTHON bin/mn -v output
-- sudo $PYTHON bin/mn --test pingall
-
-notifications:
-  email:
-    on_success: never
-
-# More details: https://docs.travis-ci.com/user/notifications

README.md

@@ -4,8 +4,7 @@ Mininet: Rapid Prototyping for Software Defined Networks
 
 Mininet 2.3.0a1
 
-[![Build Status][2]](https://github.com/mininet/mininet/actions)
-[![Travis Status][1]](https://travis-ci.org/mininet/mininet)
+[![Build Status][1]](https://github.com/mininet/mininet/actions)
 
 
 ### What is Mininet?
@@ -122,5 +121,4 @@ Mininet to change the networking world!
 Bob Lantz
 on behalf of the Mininet Contributors
 
-[1]: https://travis-ci.org/mininet/mininet.svg?branch=master
-[2]: https://github.com/mininet/mininet/workflows/mininet-tests/badge.svg
+[1]: https://github.com/mininet/mininet/workflows/mininet-tests/badge.svg