Replace Travis CI with GitHub Actions

[JRaspass]

This is a first pass at adding GitHub Actions to this project to replace the dead Travis CI. It runs and passes the test suite on every Perl release from 5.8 to 5.38.

However getting Perl 5.8, 5.10, 5.12, and to a lesser extent 5.14 working was a pain in the ass. The world has moved on and many of our direct and transitive dependencies don't support decade-and-a-half old Perls, who'd've thunk it!? My solution is to install a specific set of dependencies at specific versions:

# Install specific versions of deps that no longer support Perl 5.8
#
# Data::OptList             > 0.114  requires Perl v5.12
# Getopt::Long::Descriptive > 0.105  requires Perl v5.10
# IO::Socket::IP            > 0.41   requires Perl v5.14
# Plack                     > 1.0050 requires Perl v5.12
# Pod::Man                  > 4.14   requires Perl v5.10
# String::RewritePrefix     > 0.008  requires Perl v5.12
# Sub::Expoter              > 0.990  requires Perl v5.12
# Test::Deep                > 1.130  requires Perl v5.12
- run: >
    cpanm --notest
    Data::OptList@0.113
    Sub::Exporter@0.990
    String::RewritePrefix@0.008
    Pod::Man@4.14
    Getopt::Long::Descriptive@0.105
    IO::Socket::IP@0.41
    Test::Deep@1.130
    Plack@1.0050

While this technically works it's hella fragile and will require more and more maintenance over time as the world moves on. So I suggest we embrace this new world order and bump our minimum Perl to at least v5.12 (IO::Socket::IP is the only v5.14+ dep so far).

Bumping our Perl would allow for some nice cleanups and modernisations like using defined-or, droping Class::C3::Adopt::NEXT/MRO::Compat in favour of the core mro, and probably many others. Thoughts?

[haarg]

I would rather just stop testing on older perls, but not explicitly require a newer version.

[JRaspass]

Fair enough, I've amended the PR to only test 5.14+ and ported the rest of the logic from the old .travis.yml file. I've even managed to make the commented out reverse deps pass by disabling parallel testing.