Pin minimum supported rust version in .travis.yml

[brendanzab]

It's hard to know what minimum supported rust version is used for this crate! This means that I'm not sure what rust version to bump to - at the moment I'm getting the following build failure when building against Rust 1.42:

error: attributes are not yet allowed on `if` expressions
##[error]   --> /home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/flexi_logger-0.15.11/src/logger.rs:163:9
    |
163 |         #[cfg(feature = "colors")]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^

It would be really useful for folks downstream if an explicit version was pinned in the .travis.yml - at the moment it's only tested against stable, beta, and nightly.

[brendanzab]

Seems like the magic version is 1.43.0! It was added as a feature in rust-lang/rust#69201. So to test this as the new MSRV you just need:

rust:
  - 1.43.0
  - stable
  - beta
  - nightly

[emabee]

Thanks for this proposal! I will provide a new version along that idea.

In fact, I'm trying to be compatible down to rust 1.37.0, but will need some more time to get the travis stuff fixed. Unfortunately, one of the tests doesn't compile with 1.37.0, so I will need a more sophisticated travis config.

[brendanzab]

Cool! Yeah it can be nice to make sure you don't accidentally add things that introduce version regressions.

Thanks for the great work on this library anyway! 😀