fix skipping async validator will always fail if condition is un-fulfilled

If the validator configured with `{async:true}` option and `if/unless`
condition, validator should be skipped when the condition is un-fulfilled,
so the validator should be pass.

But currently, when skipping the validator, it calls `done(true)` which
accepts a `fail` flag as a param, this will fail the entire validation.

Signed-off-by: Clark Wang <clark.wangs@gmail.com>
Merge pull request #354 from clarkorz/fix/skip-validation

fix skipping async validator will always fail if condition is un-fulfilled