When serializing with YAML, always use the 'syck' engine.

For signing to work correctly, clients and servers must all serialise
messages in the same way.  Different versions of Ruby have different
default YAML engines, which don't have that property.  To ensure
interoperability, this change always uses the 'syck' engine (default
in 1.8).
Merge remote-tracking branch 'zts/yaml-interop-fix'

* zts/yaml-interop-fix:
  When serializing with YAML, always use the 'syck' engine.
Bump plugin version to 1.2.