README: remove travis

README.md

 libgit2 - the Git linkable library
 ==================================
 
-[![Travis Build Status](https://secure.travis-ci.org/libgit2/libgit2.svg?branch=master)](http://travis-ci.org/libgit2/libgit2)
 [![VSTS Build Status](https://libgit2.visualstudio.com/libgit2/_apis/build/status/libgit2)](https://libgit2.visualstudio.com/libgit2/_build/latest?definitionId=7)
 [![Coverity Scan Build Status](https://scan.coverity.com/projects/639/badge.svg)](https://scan.coverity.com/projects/639)