Support subscription sets [ch16280]

[tommycarney]

This PR allows for support for subscription sets for the 1.x branch. It adds a subscription_set_external_id attribute to the following objects:

• ChartMogul\LineItems
• ChartMogul\Subscription

[pkopac]

@tommycarney pls check out the build failures.

[tommycarney]

@pkopac from what I can see Travis now uses a new default build, meaning the php 5.5 install fails.

This issue is resolved if we set the distribution to trusty in the .travis.yml file.

When I do this, the nightly build fails because of the composer.json requirements don’t match.

It will work, however, when I add this snippet from master to .travis.yml to force it to ignore the restrictions:

  - |
    if [[ "$TRAVIS_PHP_VERSION" == 'nightly' ]]; then
      #ignore php version check on nightly build
      composer install --ignore-platform-reqs
    else
      composer install
    fi