Merge branch 'hotfix/2.0.1'
Merge branch 'release/2.1.0'
Merge branch 'release/2.1.1'
Merge branch 'release/2.1.2'
Merge branch 'release/2.1.3'
Merge branch 'release/2.2.0'
Merge branch 'release/2.2.1'
Merge branch 'release/2.3.0'
Merge branch 'release/2.3.1'
Merge branch 'release/2.3.2'
[FEATURE] Add Call#wait_for_end, which blocks until the call ends and returns its end reason
Merge branch 'release/2.3.3'
Merge branch 'develop' into feature/feature/wait_for_call_end
Merge branch 'release/2.3.4'
[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
Merge branch 'release/2.3.5'
[UPDATE] Use RubySpeech 2.0