Remove Travis-CI integration

This is admittedly the much abridged version, but...
Due to changes at Travis-CI that lead to prohibitively slow build/wait
times, we (edx) decided to switch CI/validation provider to Github
Actions [1].

While we had originally intended to keep the integration available,
following Travis-CI's drop in available open-source credits, it no
longer appears to be a viable option for the community.

As such, and given our already present vendor lock-in at Github,
we recommend using Github Actions for CI/validation in this repo.

However, for those not hosting in the Github ecosystem, you can
configure your testrunner to run `make validate.ci`, for the same
effect; the Github Action is just a wrapper for this call.

- [0] Fixes: TNL-7784
- [1] #276

.travis.yml

@@ -1,10 +0,0 @@
-language: node_js
-node_js: 12
-before_install:
-- npm install -g npm@6
-install:
-- npm ci
-script:
-- make validate
-after_success:
-- codecov

README.rst

@@ -1,4 +1,4 @@
-|Build Status| |Coveralls| |npm_version| |npm_downloads| |license|
+|Coveralls| |npm_version| |npm_downloads| |license|
 
 frontend-app-learning
 =========================
@@ -10,8 +10,6 @@ Introduction
 
 React app for edX learning.
 
-.. |Build Status| image:: https://api.travis-ci.org/edx/frontend-app-learning.svg?branch=master
-   :target: https://travis-ci.org/edx/frontend-app-learning
 .. |Coveralls| image:: https://img.shields.io/coveralls/edx/frontend-app-learning.svg?branch=master
    :target: https://coveralls.io/github/edx/frontend-app-learning
 .. |npm_version| image:: https://img.shields.io/npm/v/@edx/frontend-app-learning.svg