ci: update Travis badge in README.md to point to travis-ci.com

README.md

 # <img src="https://cdn.rawgit.com/spack/spack/develop/share/spack/logo/spack-logo.svg" width="64" valign="middle" alt="Spack"/> Spack
 
-[![Build Status](https://travis-ci.org/spack/spack.svg?branch=develop)](https://travis-ci.org/spack/spack)
+[![Build Status](https://travis-ci.com/spack/spack.svg?branch=develop)](https://travis-ci.com/spack/spack)
 [![Linux Builds](https://github.com/spack/spack/workflows/linux%20builds/badge.svg)](https://github.com/spack/spack/actions)
 [![macOS Builds (nightly)](https://github.com/spack/spack/workflows/macOS%20builds%20nightly/badge.svg?branch=develop)](https://github.com/spack/spack/actions?query=workflow%3A%22macOS+builds+nightly%22)
 [![codecov](https://codecov.io/gh/spack/spack/branch/develop/graph/badge.svg)](https://codecov.io/gh/spack/spack)