OSL v1.11.12.0 release

Larry Gritz

We have tagged v1.11.12.0 (a.k.a. Release- as the latest supported production release, fully back compatible with earlier 1.11.x releases.

This is a minor change that only changes some things related to build and CI. If you are not experiencing any problems that are addressed by these changes, there is certainly no rush to upgrade.

Release 1.11.12 -- 1 Mar 2021 (compared to 1.11.11)
* Build: Changes for smooth compile against upcoming Imath 3.0. #1332 #1341
* Build: We weren't properly hiding non-public symbols. #1337
* CI: Cleanup and get rid of the last Travis vestiges. #1334
* CI: Speed up CI runs by using GHA cache + ccache. #1335
* CI: Separate CI process into several individual "steps." #1338

Larry Gritz