Lock down version of phing because 2.16 onwards requires php 5.5
Lock down version of phing because 2.16 onwards requires php 5.5

- We still support php 5.3
- Phing 2.16 only requires php 5.2 but its dependency symfony/yaml ^3.1
  requires php 5.5.9
- Phing dev-master also currently conflicts with our required version
  of phpunit. We can't upgrade phpunit yet because phpunit 5 requires
  php 5.6 and phpunit 6 requires php 7

opentok/OpenTok-PHP-SDK