Should warn when processing a higher minor version YAML file

[Paalon]

According to https://yaml.org/spec/1.2.2/, 6.8.1,

Documents with a “YAML” directive specifying a higher minor version (e.g. “%YAML 1.3”) should be processed with an appropriate warning.

But does not warn.

julia> YAML.load("""%YAML 1.3\n---\nhello: world""")
Dict{Any, Any} with 1 entry:
  "hello" => "world"

[kescobo]

Related to #133