3.x: Update badge in README.md (#7115)

Author: akarnokd

README.md

@@ -1,6 +1,6 @@
 # RxJava: Reactive Extensions for the JVM
 
-<a href='https://travis-ci.com/ReactiveX/RxJava/builds'><img src='https://travis-ci.com/ReactiveX/RxJava.svg?branch=3.x'></a>
+<a href='https://github.com/ReactiveX/RxJava/actions?query=workflow%3ASnapshot'><img src='https://github.com/ReactiveX/RxJava/workflows/Snapshot/badge.svg'></a>
 [![codecov.io](http://codecov.io/github/ReactiveX/RxJava/coverage.svg?branch=3.x)](https://codecov.io/gh/ReactiveX/RxJava/branch/3.x)
 [![Maven Central](https://maven-badges.herokuapp.com/maven-central/io.reactivex.rxjava3/rxjava/badge.svg)](https://maven-badges.herokuapp.com/maven-central/io.reactivex.rxjava3/rxjava)