This is similar to the fix of #822.
Also update README to cover Mac installation.
This rolls back the cmake minimum version bump in #975 since it was
backwards-incompatible, and the following #984. Not expected to cause
any new user-facing regressions, but expected to solve any that were
introduced with #975.
* New useful getExampleNumber methods (for testing) - one for invalid
numbers and one for getting a number of a particular type without
specifying a country.