Mention #673 in History

Issue #673 adds a test for S3 and rubygems.org certificates to rubygems
so we can more quickly detect when a certificate's roots change.
Only run CA tests on travis-ci

The CA tests added in #673 run locally even when I don't have internet,
which can make the tests slower.

Now they'll only run on travis-ci which can take all the time it likes,
but will give similar value.