Make sure we support several recent versions of the toolchain

So looks like we no longer support Rust 1.12.0, because it does not export workspace_members in the metadata. By itself it is totally fine, 1.12.0 is oldish.

However, this indicates a problem: we don’t run CI over older version of toolchain, so such regressions can slip unnoticed.

@alygin @mkaput how many toolchains should we support? Looks like at the moment we support 1.13 and 1.14 and, given that 1.15 is really soon, the natural option would be to settle to three latest released minor versinos and the current nightly.

I think we should update travis such that it executes two builds: EAP IDEA + nightly rust and old IDEA + old stable rust. Does that sound reasonable?

[matklad]

Ok, let’s make it minimum two releases then! (I’ve alreday updated .travis.yml). Also, I won’t be actively updating the lower bound with every release, only if there’s some code in plugin to explicitly support older versions which can be dropped, so in practice we may support more versions!

[johnthagen]

I think this sounds reasonable, and I’ve seen other well maintained libraries provide similar compatibility support, such as clap

clap will officially support current stable Rust, minus two releases, but may work with prior releases as well. For example, current stable Rust at the time of this writing is 1.13.0, meaning clap is guaranteed to compile with 1.11.0 and beyond. At the 1.14.0 release, clap will be guaranteed to compile with 1.12.0 and beyond, etc.