gem install fails with invalid option: --no-ri

[php-coder]

ERROR:  While executing gem ... (OptionParser::InvalidOption)
    invalid option: --no-ri

Seen there: https://travis-ci.org/github/php-coder/mystamps/jobs/702825823

[php-coder]

We should use --no-document instead. See https://stackoverflow.com/questions/57475842/what-to-use-instead-of-no-ri-for-gem-install and rubygems/rubygems#2354