Migrate from travis-ci.org to travis-ci.com.

This commit fixes the URLs to Travis CI, following up on the migration that
happened recently.

README.rst

@@ -98,10 +98,10 @@ References
 .. _Iterative Ranking from Pair-wise Comparison:
  https://papers.nips.cc/paper/4701-iterative-ranking-from-pair-wise-comparisons.pdf
 
-.. |build-status| image:: https://travis-ci.org/lucasmaystre/choix.svg?branch=master
+.. |build-status| image:: https://travis-ci.com/lucasmaystre/choix.svg?branch=master
  :alt: build status
  :scale: 100%
- :target: https://travis-ci.org/lucasmaystre/choix
+ :target: https://travis-ci.com/lucasmaystre/choix
 
 .. |coverage| image:: https://codecov.io/gh/lucasmaystre/choix/branch/master/graph/badge.svg
  :alt: code coverage