Support listen v1 and v2

[twalpole]

I believe this should add support for listen v2.x while maintaining compatability with v1.x

[Wolfr]

I'm running into a dependency problem with Jekyll, @twalpole is there a way to use your gem code?

Bundler could not find compatible versions for gem "listen":
  In Gemfile:
    fontcustom (~> 1.3.3) ruby depends on
      listen (~> 1.0) ruby
    jekyll (~> 2.0) ruby depends on
      listen (2.7.9)

[yegor256]

+1 see http://stackoverflow.com/questions/26082556/how-to-force-a-version-in-bundle-gemfile