Add support for travis-ci.com

[djmcgill]

As per this blog post: travis-ci is migrating users away from travis-ci.org to travis-ci.com but this repo only supports the former.

I've added support for an optional key tld (with possible values "org" or "com") to the travis badge which will generate badge urls for pointing to .org or .com. Any other value will be a panic.

Open questions:

• Should there be a better mechanism than a panic?
• How does crates.io handle the travis-ci badge, do they have support for .com?
• I have not yet updated the docs for this change
• Bikeshed on the key "tld". Should it be added to, e.g. "service" to specify the full url?

[livioribeiro]

Does the badges work if we just replace .org with .com? If yes, then we can just use .com