Update imageasset for master Vich

As from dd408d in the master of Vich Uploader Bundle, the field name in
asset is changed to mappingName. See
https://github.com/dustin10/VichUploaderBundle/commit/dd408d554ec97ed8afcbd6ce96201982d04a5b03.
Merge pull request #145 from bobvandevijver/master

Update imageasset for master Vich