[UPDATE] Function correctly with Punchblock 2.0 headers interface

* Need to maintain BC with regards to symbol access and downcasing/substitution of dashes
[BUGFIX] Deal with non-string header names
[CS] Punchblock 2.0 stuff is now on its' develop
[CS] No need to serialise SSML output documents here
[UPDATE] Use RubySpeech 2.0
[UPDATE] Use PB 2.0's call-uri attribute
Merge branch 'feature/punchblock_2.0' into develop