Update Composer dependencies to latest (2019-04-08)

[danielbachhuber]

Loading composer repositories with package information
Updating dependencies (including require-dev)
Package operations: 2 installs, 18 updates, 0 removals
  - Updating robrichards/xmlseclibs (3.0.2 => 3.0.3): Downloading (100%)
  - Updating onelogin/php-saml (v3.0.0 => 3.1.1): Downloading (100%)
  - Updating behat/gherkin (v4.5.1 => v4.6.0): Downloading (100%)
  - Updating symfony/class-loader (v3.4.15 => v3.4.24): Downloading (100%)
  - Updating symfony/polyfill-ctype (v1.9.0 => v1.11.0): Loading from cache
  - Updating symfony/filesystem (v4.1.4 => v4.2.5): Downloading (100%)
  - Updating symfony/config (v4.1.4 => v4.2.5): Downloading (100%)
  - Updating symfony/polyfill-mbstring (v1.9.0 => v1.11.0): Loading from cache
  - Installing symfony/contracts (v1.0.2): Loading from cache
  - Updating symfony/console (v4.1.4 => v4.2.5): Downloading (100%)
  - Updating symfony/dependency-injection (v4.1.4 => v4.2.5): Downloading (100%)
  - Updating symfony/event-dispatcher (v4.1.4 => v4.2.5): Downloading (100%)
  - Updating symfony/translation (v4.1.4 => v4.2.5): Downloading (100%)
  - Updating symfony/yaml (v4.1.4 => v4.2.5): Downloading (100%)
  - Updating symfony/css-selector (v3.4.15 => v3.4.24): Downloading (100%)
  - Updating symfony/dom-crawler (v4.1.4 => v4.2.5): Loading from cache
  - Updating symfony/browser-kit (v4.1.4 => v4.2.5): Downloading (100%)
  - Installing ralouphie/getallheaders (2.0.5): Loading from cache
  - Updating guzzlehttp/psr7 (1.4.2 => 1.5.2): Loading from cache
  - Updating pantheon-systems/pantheon-wordpress-upstream-tests dev-master (12929d6 => ef6b222):  Checking out ef6b222225
symfony/contracts suggests installing psr/cache (When using the Cache contracts)
symfony/contracts suggests installing symfony/cache-contracts-implementation
Writing lock file
Generating autoload files

Fixes #138