undefined method `plain_specs' for #<Bundler::RubygemsIntegration:0x00005633f09a9000> (NoMethodError)

gem uninstall rubygems-bundler

works for me I am using bundler 2.1.0

REF: https://github.com/bundler/bundler/issues/7492

When showing

ERROR:  While executing gem ... (Gem::InstallError)
    rubygems-bundler is not installed in GEM_HOME, try:
    gem uninstall -i /Users/xxxx/.rvm/rubies/ruby-2.5.7/lib/ruby/gems/2.5.0 rubygems-bundler

Please run

gem uninstall -i /Users/xxxx/.rvm/rubies/ruby-2.5.7/lib/ruby/gems/2.5.0 rubygems-bundler