Support JSONSchema.jl beyond v1.4

[matthijscox]

In PR4 we found that a test fails for JSONSchema 1.4.1 and up. For now we restrict the compact to <1.4.1 but we should figure out a proper solution.

Here's the test log snippet:

Nested Structs |    4      4  0.2s
Test Summary:                  | Pass  Total  Time
StructTypes.DataType gathering |    6      6  0.1s
Nested Structs using schema references: Error During Test at /JSONSchemaGenerator/test/runtests.jl:96
  Test threw exception
  Expression: JSONSchema.validate(my_schema, JSON.parse(json_string)) === nothing
  MethodError: no method matching _validate_entry(::Dict{String, Any}, ::String, ::String)
  The function `_validate_entry` exists, but no method is defined for this combination of argument types.

Would be great if someone could debug to figure out whether:

• this test scenario is wrong or irrelevant, or
• JSONSchemaGenerator.jl has a bug, or
• JSONSchema.jl has a bug