Don't change the default gem directory.
    
    Ruby expects default gem specifications to be stored in a subdirectory
    of default gem specifications directory. Change in default directory
    makes the default gems unloadable.
    
    Unfortunately, when '--install-dir' option is used, this might prevent
    generating of documentation [1].
    
    [1] https://github.com/rubygems/rubygems/issues/1470