travisci badge should use `.com` url instead of `.org`

[robturtle]

According to https://stackoverflow.com/a/51656568/2214973, the travis-ci.org is deprecated. We should use travis-ci.com for the badge url instead.

[balupton]

For repos that have not yet been migrated to travis-ci.com, org still remains appropriate.

Once travis-ci has migrated all repos over, which remains unspecified (I've sent them an email asking when), then swapping the default from com to org would be appropriate. The default is specified in badges.

In the meantime, you can make use of a "travisTLD": "com" badge configuration.

If you use boundation such migration and configuration determination is applied automatically.