Make compatible with Middleman v3 and v4

[manuelmeurer]

After merging #23 (and doing some cleanup), the master branch currently only works with Middleman v4. I would like it to work with Middleman v3 and v4 and then release 1.0. 😄
It shouldn't be too hard, we probably only need to make sure to use the v3 syntax for loading the Extension and Invalidate classes (see lib/middleman-bootstrap-navbar/extension.rb here)
First I would like to add Appraisal to test different Middleman versions and started to do so in the appraisals branch.
Since I'm no good at Cucumber, I'm having a hard time writing the cukes though. 😄
If someone could help me out here, that would be fantastic.
@andrusha do you have time?

[monfresh]

I had been using both middleman-s3_sync and this gem, but with the after_sync being removed in Middleman v4, and out of curiosity, I started looking for alternatives. I came across a gem that does not have any dependency on any specific static site generator. This means you can use it with any version of Middleman or Jekyll or any other generator. Check it out, it's really cool:

https://github.com/laurilehmijoki/s3_website

It supports S3 sync, CF invalidation, redirects, and a lot more.

[manuelmeurer]

@monfresh Looks interesting, thanks for the hint! 😄