API Defaulting composer core dependencies to to @stable

Required to ensure correct operation of
"composer create-project silverstripe/installer my-path 2.4.10"
(checkout with tags).
Removed @stable markers from 2.4 dev branch
FIX Include flushtoken when install redirects to successfullyinstalled
Merge pull request #47 from hafriedlander/fix/flush_24

FIX Include flushtoken when install redirects to successfullyinstalled
Merge remote-tracking branch 'origin/2.4' into 3.0