Bump Middleman to v5.0.0

[joshRpowell]

Noticed there is a RC as of 5/24/19: middleman/middleman@v4.3.4...v5.0.0.rc.1

[neetso]

Anyone know which version of the contentful_middleman gem I should lock to if I'm staying on middleman 4.2 for the time being?

[dlitvakb]

Hey Matt Steen (@neetso),

You can stay in contentful_middleman 4.x.x, I've decided to keep the major numbers in line with Middleman's versioning.

Josh Powell (@joshRpowell), thanks for the PR, I'll look into adding support for the 5.x version as soon as it gets an official release.

Cheers