Use DUB 1.2.1 for testing with AppVeyor.
Use DUB 1.2.1 for testing with AppVeyor.

This avoids the "trying to append absolute path" error when testing sub packages.

DBankov/vibe.d,rejectedsoftware/vibe.d,etcimon/vibe.d,gedaiu/vibe.d,Geod24/vibe.d,etcimon/vibe.d,chalucha/vibe.d