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

@@ -68,14 +68,14 @@
         "wpackagist-theme/thematic": "1.0.4",
         "wpackagist-theme/the-scenery": "0.93",
         "wpackagist-theme/threattocreativity": "3.2",
-        "wpackagist-theme/twentyeleven": "3.9",
+        "wpackagist-theme/twentyeleven": "4.0",
         "wpackagist-theme/twentyfifteen": "3.0",
         "wpackagist-theme/twentyfourteen": "3.2",
         "wpackagist-theme/twentynineteen": "2.2",
         "wpackagist-theme/twentyseventeen": "2.9",
         "wpackagist-theme/twentysixteen": "2.5",
         "wpackagist-theme/twentyten": "3.6",
-        "wpackagist-theme/twentythirteen": "3.4",
+        "wpackagist-theme/twentythirteen": "3.5",
         "wpackagist-theme/twentytwelve": "3.6",
         "wpackagist-theme/twentytwenty": "1.9",
         "wpackagist-theme/wp-andreas01": "2.0",