fix: ensure ruby 2.7 compatibility by including scanf gem

[hendrixfan]

fix for #7

• scanf was excluded from the standard libary as of ruby 2.7
• see https://rubyreferences.github.io/rubychanges/2.7.html#libraries-excluded-from-the-standard-library
• include the official scanf gem in gemspec

[zeevallin]

Thanks for the PR @hendrixfan, I'll take a look at this locally and see if I can do some other upgrades to keep up to par with current Ruby and make sure Travis runs properly.

[hendrixfan]

Thank you for taking a look!