ci: Remove support for Travis CI

So long, and thanks for all fish!

Author: real-or-random

.travis.yml

@@ -1,7 +1,6 @@
 libsecp256k1
 ============
 
-[![Build Status](https://travis-ci.org/bitcoin-core/secp256k1.svg?branch=master)](https://travis-ci.org/bitcoin-core/secp256k1)
 [![Build Status](https://api.cirrus-ci.com/github/bitcoin-core/secp256k1.svg?branch=master)](https://cirrus-ci.com/github/bitcoin-core/secp256k1)
 
 Optimized C library for ECDSA signatures and secret/public key operations on curve secp256k1.