CI doesn't validate human-panic still works with oldest supported rust version

[epage]

🐛 bug report

I noticed .travis.yml isn't running tests on the oldest supported version of rust. This makes it harder for users to find what version of rust is supported and makes it easy to accidentally change the oldest supported version without fanfare.

This came up when someone reported to me that they had to upgrade their version of rust to try out human-panic due to a use-statement syntax being used.

See docmatic for an example on how to enforce this.