CallableBuilder: fix for ceylon/ceylon.language#231: add variadicity info to Callable impl
CallableBuilder: clarify that isVariadic was actually about optional params, and added a proper isVariadic ceylon/ceylon.language#231
Updated test .src due to fix for ceylon/ceylon.language#231: added variadicity info to Callable impl
ceylon doc tests: use 0.6 SDK
Updated tests for binary version bump