Update dependency composer/installers to v1.12.0

This MR contains the following updates:

Package	Type	Update	Change
composer/installers (source)	require	minor	1.11.0 -> 1.12.0

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Release Notes

composer/installers

v1.12.0

Compare Source

Added

• Added installer for Pantheon Quicksilver modules/scripts #​495.

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composer.json

@@ -76,7 +76,7 @@
         "wpackagist-theme/twentysixteen": "2.5",
         "wpackagist-theme/twentyten": "3.6",
         "wpackagist-theme/twentythirteen": "3.4",
-        "wpackagist-theme/twentytwelve": "3.5",
+        "wpackagist-theme/twentytwelve": "3.6",
         "wpackagist-theme/twentytwenty": "1.8",
         "wpackagist-theme/wp-andreas01": "2.0",
         "wpackagist-theme/zenlite": "4.10",