Add missing require.

This file uses rspec-support’s MethodSignatureVerifier
but does not load it. It was working because rspec-mocks
loads it so it was loaded in the same process. It would
fail for users using rspec-expectations but not rspec-mocks,
though.
Merge pull request #566 from rspec/fix-yield-matcher-arity-check

Add missing require.
Add changelog for #566.

[ci skip]
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]
Make `contain_exactly` work with out of order hashes.

The reason that this was failing is that `contain_exactly` assumed that
its `expected` value was a matcher. As such it was using === to compare
the expected value with the actual value. This is completely correct for
matchers. However, when a hash is passed the implementation of === will
not match the actual value at all. Hash === won't invoke the RSpec
matchers.

This fix uses the lower performance values_match? which correctly
matches when hashes are provided.

Fixes #574.