Upgrade GWC dependency to 1.7-SNAPSHOT
[GEOS-6627]GWC causes a wrong handling of the ImageIO and ImageIO-Ext plugins

Conflicts:

	src/pom.xml
Merge pull request #686 from n-lagomarsini/GEOS-6627

[GEOS-6627] GWC causes a wrong handling of the ImageIO and ImageIO-Ext plugins