Bump the MW required version to 1.24wmf6

As of I87e93777 VisualEditor-MediaWiki now use the new MediaWiki core RL
module of mediawiki.skinning.content.parsoid, introduced in 1.24wmf6,
which has bumped our dependency to an alpha of 1.24.

Change-Id: I33db154980ec207edab75751d60cb4764144f32b
Merge "Bump the MW required version to 1.24wmf6"