Update bundler to 2.1.4

[pelargir]

This updates our gemspec to use a more recent version of bundler (bundler 2.1.4 is included with Ruby 2.7.2).

[mamhoff]

Thank you, but we allow older Rubies as well. The gem is compatible with older bundlers, and with newer bundlers, and I think we should keep that version window as wide as possible.

[tvdeyen]

Thank you, but we allow older Rubies as well. The gem is compatible with older bundlers, and with newer bundlers, and I think we should keep that version window as wide as possible.

FYI this is a dev dependency that only gets installed on the local machine while your writing the gem. This should not effect anybody else using the gem

[pelargir]

@tvdeyen Right. I don't see a problem with requiring Bundler 2 in this situation. Bundler 1 was last updated over 2 years ago and the only advantage it offers over 2 is access to now-deprecated Ruby and RubyGems versions.

[tvdeyen]

Yes, 2FA support for pushing to Rubygems is one it

[tvdeyen]

@mamhoff I think we should re-open this PR

[mamhoff]

Sorry, @pelargir - I did not realize we're talking about a dev dependency here.

Realizing I was wrong requesting this change

[pelargir]

Thanks!