Remove deprecated Travis CI config

Looks like getssl was not using Travis CI for a while:
https://travis-ci.org/github/srvrco/getssl

And the GitHub Actions was introduced for a while:
https://github.com/srvrco/getssl/commits/master/.github/workflows

Remove Travis CI config in the repository will help contributor reduce
the confusion about which CI platform is currently using.

.travis.yml

@@ -1,17 +0,0 @@
-language: bash
-
-# Use container-based infrastructure for quicker build start-up
-sudo: false
-
-addons:
-  apt:
-    sources:
-    - debian-sid    # Grab shellcheck from the Debian repo (o_O)
-    packages:
-    - shellcheck
-
-script:
- - bash -c 'shopt -s globstar; shellcheck getssl'
-
-matrix:
-  fast_finish: true