Disable --vendor when vendordir is missing

JRuby does not have any vendor keys in its RbConfig::CONFIG which
prevents RubyGems 2.4.0 from installing.

Part of #974
Ignore vendor_dir in default_path when missing

If there is no vendor_dir configured in the implementations
RbConfig::CONFIG then the vendor directory must be ignored in the
default path as well.

Fixes #974