Replace Travis for continuous integration with Github Actions.
The Github Actions pipeline is functionally equivalent, with
all the same build permutations supported.
Whilst the Github Actions offering is broadly equivalent to
Travis, a few changes have been made as part of the migration.
- The 'core' and 'optional' Travis stages have been consolidated
into one build matrix. This is due to the current inability in
Github Actions to share build steps between jobs, so this avoids
having to duplicate the steps.
Optional stage jobs will now run in parallel with core jobs,
but they still remain optional in the sense that they don't fail
the build.
- A number of existing Github Action plugins are used to replace
functionality provided by Travis or other tools:
Node setup, caching, Codecov, publishing release artifacts.
- Linux builds are updated to build on Ubuntu 18.04.
MacOS builds are updated to run on 10.15. Similar to the
Travis Xenial upgrade attempt, some changes are required due
to underlying platform and compiler upgrades. This means some
Node 10 toolchains will no longer be supported.
Whilst there is opportunity to upgrade some dependencies and
make the CI steps more idiomatic, I've left this for future changes
and just focussed on functional replication.
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