Document PHP7 requirement

[stevector]

Running the setup script I got this message:

./bin/local-build.sh

Invoking: /usr/local/bin/composer install --no-ansi --no-interaction --optimize-autoloader --no-progress --no-scripts 
Loading composer repositories with package information
Installing dependencies (including require-dev) from lock file
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Installation request for ocramius/package-versions 1.1.2 -> satisfiable by ocramius/package-versions[1.1.2].
    - ocramius/package-versions 1.1.2 requires php ~7.0 -> your PHP version (5.6.20) does not satisfy that requirement.
  Problem 2
    - Installation request for ocramius/proxy-manager 2.1.x-dev -> satisfiable by ocramius/proxy-manager[2.1.x-dev].
    - ocramius/proxy-manager 2.1.x-dev requires php ^7.1.0 -> your PHP version (5.6.20) does not satisfy that requirement.
  Problem 3
    - ocramius/proxy-manager 2.1.x-dev requires php ^7.1.0 -> your PHP version (5.6.20) does not satisfy that requirement.
    - sensiolabs/behat-page-object-extension dev-master requires ocramius/proxy-manager ^1.0||^2.0 -> satisfiable by ocramius/proxy-manager[2.1.x-dev].
    - Installation request for sensiolabs/behat-page-object-extension dev-master -> satisfiable by sensiolabs/behat-page-object-extension[dev-master].

I think a requirement for PHP7 should go in the README.

[tobeycodes]

Ideally we should update this line

https://github.com/ataylorme/Advanced-WordPress-on-Pantheon/blob/master/composer.json#L45