forcing version 1.2 of assetmanager, since 1.3 is broken.
Merge pull request #541 from Technigami/master

forcing version 1.2 of assetmanager, since 1.3 is broken.
Needed to add assetmanger 0.1.2 hardcoding to devDependencies as well
#536 use required instead of validate for simple existence check
dependencies need only be in one group

`assetmanager` does not need to be under both `dependencies` and `devDependencies`. Removed from the latter.