Yes, I know 1.9.3 has been EOL'd many months ago. It is OK if this is a carefully considered decision. But https://github.com/jwt/ruby-jwt/blob/v1.5.3/ruby-jwt.gemspec does not indicate it.
It should be either indicated as such, or reinstate 1.9.x support.
Upon require, it fails:
$ pry
[1] pry(main)> require 'jwt'
SyntaxError: /Users/asari/.rvm/gems/ruby-1.9.3-p551/gems/jwt-1.5.3/lib/jwt/verify.rb:7: unknown type of %string
%i[verify_aud verify_expiration ...
^
/Users/asari/.rvm/gems/ruby-1.9.3-p551/gems/jwt-1.5.3/lib/jwt/verify.rb:7: syntax error, unexpected $end
%i[verify_aud verify_expiration ...
^
from /Users/asari/.rvm/rubies/ruby-1.9.3-p551/lib/ruby/site_ruby/1.9.1/rubygems/core_ext/kernel_require.rb:54:in `require'