Update Composer depedencies (2019-09-16)

```
Loading composer repositories with package information
Updating dependencies (including require-dev)
Package operations: 0 installs, 27 updates, 0 removals
  - Updating onelogin/php-saml (3.1.1 => 3.3.0): Downloading (100%)
  - Updating sebastian/exporter (3.1.0 => 3.1.2): Loading from cache
  - Updating phpunit/php-token-stream (3.0.1 => 3.1.0): Loading from cache
  - Updating symfony/polyfill-ctype (v1.11.0 => v1.12.0): Loading from cache
  - Updating webmozart/assert (1.4.0 => 1.5.0): Loading from cache
  - Updating phpdocumentor/reflection-common (1.0.1 => 2.0.0): Loading from cache
  - Updating phpdocumentor/type-resolver (0.4.0 => 1.0.1): Loading from cache
  - Updating phpdocumentor/reflection-docblock (4.3.1 => 4.3.2): Loading from cache
  - Updating myclabs/deep-copy (1.9.1 => 1.9.3): Loading from cache
  - Updating phpunit/phpunit (7.5.13 => 7.5.16): Downloading (100%)
  - Updating symfony/class-loader (v3.4.29 => v3.4.31): Downloading (100%)
  - Updating symfony/filesystem (v4.3.2 => v4.3.4): Loading from cache
  - Updating symfony/config (v4.3.2 => v4.3.4): Loading from cache
  - Updating symfony/service-contracts (v1.1.5 => v1.1.6): Loading from cache
  - Updating symfony/polyfill-php73 (v1.11.0 => v1.12.0): Loading from cache
  - Updating symfony/polyfill-mbstring (v1.11.0 => v1.12.0): Loading from cache
  - Updating symfony/console (v4.3.2 => v4.3.4): Loading from cache
  - Updating symfony/dependency-injection (v4.3.2 => v4.3.4): Loading from cache
  - Updating symfony/event-dispatcher (v4.3.2 => v4.3.4): Loading from cache
  - Updating symfony/translation-contracts (v1.1.5 => v1.1.6): Loading from cache
  - Updating symfony/translation (v4.3.2 => v4.3.4): Loading from cache
  - Updating symfony/yaml (v4.3.2 => v4.3.4): Loading from cache
  - Updating symfony/css-selector (v3.4.29 => v3.4.31): Downloading (100%)
  - Updating symfony/dom-crawler (v4.3.2 => v4.3.4): Downloading (100%)
  - Updating symfony/browser-kit (v4.3.2 => v4.3.4): Downloading (100%)
  - Updating guzzlehttp/psr7 (1.6.0 => 1.6.1): Loading from cache
  - Updating pantheon-systems/pantheon-wordpress-upstream-tests dev-master (ef6b222 => 2f25dde):  Checking out 2f25ddefdd
```