gemを使うときに変なエラーが出たら (ERROR: While executing gem ... (TypeError) incompatible marshal file format (can't be read) format version 4.8 required; 60.33 given)

下記のようなエラーが出たとき

gem install bundler
ERROR:  While executing gem ... (TypeError)
    incompatible marshal file format (can't be read)
    format version 4.8 required; 60.33 given

gem update --system
ERROR:  While executing gem ... (Gem::RemoteFetcher::FetchError)
    server did not return a valid file (http://gems.rubyforge.org/specs.4.8.gz)

解決法

sourcesが古いからだった

gem env
RubyGems Environment:
...
  - REMOTE SOURCES:
     - http://gems.rubyforge.org/
     - http://gems.github.com
...

なので、

# 下記2つのソースを削除
gem sources -r http://gems.rubyforge.org/
gem sources -r http://gems.github.com

# 追加
gem sources -a https://rubygems.org/

# 確認
gem sources -l                       
*** CURRENT SOURCES ***

https://rubygems.org/

で解決

ここでも役立っていた😁
https://github.com/rubygems/rubygems/issues/1589#issuecomment-466381653