Add argument to satisfy typechecker on which signature to use

This gets rid of a typecheck error on the solargraph codebase--the RBS
for the method called has two signatures, and without the additional
argument in the call neither signature is satisified.

It works in practice because the single implementation already
defaults this 'version' argument to nil.

lib/solargraph/rbs_map.rb

@@ -57,7 +57,7 @@ def repository
     # @return [Boolean] true if adding the library succeeded
     def add_library loader, library
       @resolved = if loader.has_library?(library: library, version: nil)
-        loader.add library: library
+        loader.add library: library, version: nil
         Solargraph.logger.info "#{short_name} successfully loaded library #{library}"
         true
       else