(PUP-3272) Remove support yaml on network

This removes YAML as a supported network format, and completely drops the format :b64_zlib_yaml.

* The safe_yaml vendored gem is removed
* As a consequence of dropping safe_yaml, the serialization API had to change slightly (Psych yaml directly sets instance variables).
* Test for roundtrip for yaml with Ruby 1.8.7 no longer works because it does not have Psych, and Ruby 1.8.7 will be dropped.
* The option :legacy_query_parameter_serialization is dropped
* The unused puppet-load application is dropped
* Request parameters in YAML are now treated as strings
* Requests and Responses containing YAML raises errors
Don't know. I kept it since we may add support for other formats.