Incorporate default site timezone into the FBIA Transformer

Problem/Motivation

In the following pull request, Facebook added support for specifying a timezone in the Transformer. It's used by the TimeRule, which is a newer rule that the module doesn't use by default, but a developer might expect to use it and see it work.

https://github.com/facebook/facebook-instant-articles-sdk-php/pull/221

By default the timezone is set to PST. We should be setting it to the site wide default timezone.

Proposed resolution

Implement the constructor in \Drupal\fb_instant_articles\Transformer and pass in the site wide timezone to the parent constructor (Facebook\InstantArticles\Transformer\Transformer). Also require ~1.8.3 of FBIA PHP SDK. This will be part of other updates to be compatible with the latest version of the FBIA PHP SDK.

Remaining tasks

Implement.

User interface changes

None.

API changes

Developers will now be able to use the TimeRule effectively.

Data model changes

None.