doc: Update CI badge for Travis

... from .org to .com, since the former is shut down.

related: #137

README.adoc

@@ -4,7 +4,7 @@ ifdef::env-github[]
 image:https://img.shields.io/gem/v/enmail.svg[
 	Gem Version, link="https://rubygems.org/gems/enmail"]
 image:https://img.shields.io/travis/riboseinc/enmail/master.svg[
-	Build Status, link="https://travis-ci.org/riboseinc/enmail/branches"]
+	Build Status, link="https://travis-ci.com/riboseinc/enmail/branches"]
 image:https://img.shields.io/codecov/c/github/riboseinc/enmail.svg[
 	Test Coverage, link="https://codecov.io/gh/riboseinc/enmail"]
 image:https://img.shields.io/codeclimate/maintainability/riboseinc/enmail.svg[