Update Composer Deps

[adamhake]

Running terminus build:project:create wp <site-name> fails with the following message:

Your lock file does not contain a compatible set of packages. Please run composer update.

  Problem 1
    - composer/installers is locked to version v1.7.0 and an update of this package was not requested.
    - composer/installers v1.7.0 requires composer-plugin-api ^1.0 -> found composer-plugin-api[2.1.0] but it does not match the constraint.
  Problem 2
    - johnpbloch/wordpress-core-installer is locked to version 0.2.1 and an update of this package was not requested.
    - johnpbloch/wordpress-core-installer 0.2.1 requires composer-plugin-api ^1.0 -> found composer-plugin-api[2.1.0] but it does not match the constraint.
  Problem 3
    - rvtraveller/qs-composer-installer is locked to version 1.1 and an update of this package was not requested.
    - rvtraveller/qs-composer-installer 1.1 requires composer-plugin-api ~1.0 -> found composer-plugin-api[2.1.0] but it does not match the constraint.
  Problem 4
    - dealerdirect/phpcodesniffer-composer-installer is locked to version v0.5.0 and an update of this package was not requested.
    - dealerdirect/phpcodesniffer-composer-installer v0.5.0 requires composer-plugin-api ^1.0 -> found composer-plugin-api[2.1.0] but it does not match the constraint.
  Problem 5
    - ocramius/package-versions is locked to version 1.4.0 and an update of this package was not requested.
    - ocramius/package-versions 1.4.0 requires composer-plugin-api ^1.0.0 -> found composer-plugin-api[2.1.0] but it does not match the constraint.
  Problem 6
    - composer/installers v1.7.0 requires composer-plugin-api ^1.0 -> found composer-plugin-api[2.1.0] but it does not match the constraint.
    - wpackagist-theme/twentynineteen 1.4 requires composer/installers ~1.0 -> satisfiable by composer/installers[v1.7.0].
    - wpackagist-theme/twentynineteen is locked to version 1.4 and an update of this package was not requested.

[kporras07]

Related to #462