Remove "websocket-client-simple" dependency and "ruboty-slack_rtm" upgrade to v3.2.4 or higher

[mziyut]

What

Remove "websocket-client-simple" dependency and "ruboty-slack_rtm" upgrade to v3.2.4 or higher.

Goals

• Remove "websocket-client-simple" dependency from Gemfile
• Upgrade "ruboty-slack_rtm" from v3.2.1 to v3.2.4 or higher

Non-Goals

• Upgrade ruby version
  • Bump ruby from 2.6.3 to 2.7.6 #29

Why

"websocket-client-simple" dependency was added in #17. It is because solves the problem in "OpenSSL::SSL::SSLError: SSL_write error". (shokai/websocket-client-simple#36)
The fix has since been reflected in "ruboty-slack_rtm" and v3.2.4 has been released so that the dependency can be removed (rosylilly/ruboty-slack_rtm#47). However, Ruby 2.6.7 or higher is required to incorporate the changes.

References

• Fix OpenSSL::SSL::SSLError: SSL_write error / SNI support  #17
• Added to use SNI shokai/websocket-client-simple#36
• rescue when OpenSSL::SSL::SSLError raised ruby-jp/websocket-client-simple#10
• OpenSSL::SSL::SSLError: SSL_write at Heroku rosylilly/ruboty-slack_rtm#46
• Use websocket-client-simple gem v0.5.0 or greater rosylilly/ruboty-slack_rtm#47