Pin to Celluloid 0.15.x due to possible incompatibility with Celluloid 0.16

Pending investigation
[BUGFIX] Fix possible infinite blocking

Avoid cases where executing AGI commands (reported as sending SIP messages) might block indefinitely due to a race condition in fetching their response value.

Requires a fix to Celluloid: https://github.com/celluloid/celluloid/issues/456
Merge branch 'feature/reliable-agi-response-wait' into develop