Call to undefined function gmp_init

[bshaffer]

Following the instructions, I received this error:

PHP Fatal error:  Call to undefined function gmp_init() in /Library/WebServer/Code/protoc-gen-php/protocolbuffers.inc.php on line 285

Fix:

brew install php56-gmp

Suggestion:

• include the GMP extension as part of the requirements
• Check this extension is enabled and throw a useful error before attempting to install
• provide extension install instructions for mac/linux