Update php minimum requirement

[skippednote]

composer install fails on PHP 5.5.29 on running .blt.sh configure as some of the dependencies require 5.6.

❯ ./blt.sh configure
Phing was not found in this project's bin directory.
Attempting to run composer install. This takes a few minutes.
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 phpunit/php-code-coverage 4.0.0 -> satisfiable by phpunit/php-code-coverage[4.0.0].
    - phpunit/php-code-coverage 4.0.0 requires php ^5.6 || ^7.0 -> your PHP version (5.5.34) does not satisfy that requirement.
  Problem 2
    - Installation request for phpunit/phpunit 5.4.2 -> satisfiable by phpunit/phpunit[5.4.2].
    - phpunit/phpunit 5.4.2 requires php ^5.6 || ^7.0 -> your PHP version (5.5.34) does not satisfy that requirement.
  Problem 3
    - Installation request for phpunit/phpunit-mock-objects 3.2.1 -> satisfiable by phpunit/phpunit-mock-objects[3.2.1].
    - phpunit/phpunit-mock-objects 3.2.1 requires php ^5.6 || ^7.0 -> your PHP version (5.5.34) does not satisfy that requirement.
  Problem 4
    - Installation request for sebastian/code-unit-reverse-lookup 1.0.0 -> satisfiable by sebastian/code-unit-reverse-lookup[1.0.0].
    - sebastian/code-unit-reverse-lookup 1.0.0 requires php >=5.6 -> your PHP version (5.5.34) does not satisfy that requirement.
  Problem 5
    - Installation request for sebastian/object-enumerator 1.0.0 -> satisfiable by sebastian/object-enumerator[1.0.0].
    - sebastian/object-enumerator 1.0.0 requires php >=5.6 -> your PHP version (5.5.34) does not satisfy that requirement.
  Problem 6
    - Installation request for sebastian/resource-operations 1.0.0 -> satisfiable by sebastian/resource-operations[1.0.0].
    - sebastian/resource-operations 1.0.0 requires php >=5.6.0 -> your PHP version (5.5.34) does not satisfy that requirement.
  Problem 7
    - Installation request for sebastian/version 2.0.0 -> satisfiable by sebastian/version[2.0.0].
    - sebastian/version 2.0.0 requires php >=5.6 -> your PHP version (5.5.34) does not satisfy that requirement.

./blt.sh: line 23: /Users/bassam/Desktop/blt/vendor/bin/phing: No such file or directory