Use maintenance-branch in Gemfile.
Merge pull request #569 from rspec/use-maint-branch-master

Use maintenance-branch in Gemfile.
dont treat structs as arrays when generating descriptions
changelog for #576
[skip ci]
Merge pull request #576 from rspec/fix_diffs_for_structs

Don't treat Structs as enumerable when generating description.
Fix constant casing.
Port changelog from 3.0.1 release.

[ci skip]
Add changelog entry for #563.

[ci skip]
Fix arity check for yield matchers.
Fix warnings produced by output matcher.
Support compound block expectations.

For #552.