Freeze Flow and Echo extensions for group2 wikis

Flow team doesn't want 1.24wmf18 versions to roll out to enwiki in
Aug 28 release train.

So keep Flow at 1.24wmf17, and keep Echo at 1.24wmf16 (it was earlier
rolled back to this in 1.24wmf17 by change Id4ecf2184842).

Change-Id: Ieb90138a18628ceabbe70d82c0151da8ac5a4111