target platform resolution adapted to new xtext update site

See http://www.eclipse.org/forums/index.php/t/791479/

Change-Id: I100d85a84fa51557fb749b9b7d902c8a9e1783e0
Temporarily ignore failing tests for Xtext 2.6.0

Change-Id: I739c21fe930fb6b78ec483130a00dae37939a539
min version 1.4.0 for xpand

Otherwise the generated bundles would expose an automatic version range
[1.4.0,2.0.0) which prevented installation in Luna

Change-Id: I0719b5366d2b95fcb4e458d6f41b8934c1dc3b13
Updated aggregator file

Change-Id: Idf6e24caf9f565febdce3a8a2471fdbdd36524c9