Update supported compiler versions ...

Update LDC used for release from v1.20 to v1.22,
move coverage target to be the latest compiler
(so hopefully the less buggy too),
and make sure we only support the last 10 releases.

Travis,deploy: Update list of supported compilers ...

Build with the latest LDC, which doesn't have the stack overflow in GC bug.

Move most CI scripts in scripts/ci ...

The root directory was getting a bit crowded.
semaphore-ci.sh was not moved, because there's no visible way to configure semaphore from the repository.
Additionally, build.d is left in the root directory as it's useful to have it here,
and it'll be only one script one build.sh, build.cmd, and build-gdc.sh are gone.