Update Travis link to use .com domain (#325)

The Travis repository has already been migrated to the .com domain,
all that remains is to update this link.

In general, this change as announced by Travis in May 2018:
https://docs.travis-ci.com/user/migrate/open-source-on-travis-ci-com/

README.md

@@ -1,4 +1,4 @@
-[![Build Status](https://travis-ci.org/mcoblenz/Obsidian.svg?branch=master)](https://travis-ci.org/mcoblenz/Obsidian)
+[![Build Status](https://travis-ci.com/mcoblenz/Obsidian.svg?branch=master)](https://travis-ci.com/mcoblenz/Obsidian)
 [![Documentation Status](https://readthedocs.org/projects/obsidian/badge/?version=latest)](https://obsidian.readthedocs.io/en/latest/?badge=latest)