corber/cordova/www/.gitkeep deleted when building

[DuBistKomisch]

Seems to be introduced in 1.3.3 with 05b571a2 since it now "empties" the output directory.

[alexblom]

Thanks! We started deliberately emptying the directory for a corber build/output as we found people were not re-building. Given our builds include mobile assets it was causing some confusion.

Perhaps we should re-evaluate that, but in the interim whitelist .gitkeep for deletion?