README: Remove Travis CI badge

Bye, bye Travis CI:
" We are unable to start your build at this time. You exceeded the number
of users allowed for your plan. Please review your plan details and
follow the steps to resolution. "

github Actions is doing the job now.

README.md

@@ -17,8 +17,6 @@ Low level API samples: See https://pyscard.sourceforge.io/pyscard-wrapper.html#w
 
 Continuous Integration status
 -----------------------------
-Travis CI: [![Build Status](https://travis-ci.com/LudovicRousseau/pyscard.svg?branch=master)](https://travis-ci.com/LudovicRousseau/pyscard)
-
 AppVeyor: [![Build status](https://ci.appveyor.com/api/projects/status/c97dsaodpcwkm0ra?svg=true)](https://ci.appveyor.com/project/LudovicRousseau/pyscard)
 
 Coveralls: [![Coverage Status](https://coveralls.io/repos/LudovicRousseau/pyscard/badge.svg?branch=master&service=github)](https://coveralls.io/github/LudovicRousseau/pyscard?branch=master)