Port to current `bats-core`

[rico-chet]

The original bats repository is abandoned (no commits for years), the successor being bats-core. Over years, its development progressed, yet bats-file didn't follow, so now tests are failing due to a couple internal changes.

Luckily, fixing the tests is all that's needed, business logic isn't affected by the dependency upgrade.

To ensure backward compatibility to the old bats, additional travis environments were added so that a bunch of bats-core versions are tested at once, v0.4.0 being same as the latest bats release and the oldest version to be supported.