Test with gdc-7 and gdc-8

[wilzbach]

See also:

• Update to dmd master 20180824 D-Programming-GDC/gdc#704 (comment)
• Remove contracts from StaticForEach #8540 (recent removal of in contracts in DMD for GDC-7 support)

CC @ibuclaw

I already configured another gdc-7 job at SemaphoreCI.
With the free plan we can only run four jobs in parallel, but this seems to be important to the GDC project and also the long-term plan is to migrate all SemaphoreCI tests to Buildkite runners (e.g. dlang/ci#261) where we can just arbitrarily (and cheaply!) add new runners (we already have ten agents there).

I know that this way of hacking with the Ubuntu PPA isn't ideal, but as long as the install.sh doesn't support newer GDC binaries, this is an easy workaround.

[dlang-bot]

Thanks for your pull request, @wilzbach!

Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + dmd#8565"

[ibuclaw]

To clarify my stance, I think it is a reasonable request to ensure that the last two releases of gdc work with compiling dmd. When gdc-9 is out, you can drop gdc-7.