(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
If we ever want to add other formats we can put something back in. If it isn't being used it should just go away.