Support activesupport 5.x series

[minimum2scp]

activesupport 5.0, 5.1 and 5.2 has String#{demodulize,underscore} as same as 4.2.7

• 4.2.7
  • String#demodulize
  • String#underscore
• 5.0.7
  • String#demodulize
  • String#underscore
• 5.1.6
  • String#demodulize
  • String#underscore
• 5.2.0
  • String#demodulize
  • String#underscore

so it seems no problem to update gem dependency.

[gongo]

Thanks for PR 🍕

[gongo]

@minimum2scp I've released 0.7.1 🎉

[minimum2scp]

Thanks!