gem old dependency

[RealSilo]

Hey @andrusha ! Thanks for the gem! First time using it and looks awesome, although I ran into this issue. Could you pls update the dependencies? Thanks a lot!

Bundler could not find compatible versions for gem "middleman-core":

  In snapshot (Gemfile.lock):

    middleman-core (= 4.1.9)

  In Gemfile:

    middleman (>= 4.0.0) was resolved to 4.1.9, which depends on
      middleman-core (= 4.1.9)

    middleman-cloudfront (~> 0.2.1) was resolved to 0.2.1, which depends on
      middleman-core (~> 3.0)

    .......

[manuelmeurer]

Unfortunately this gem is not compatible with Middleman 4 yet, see #26.