Update Arg for specs to use interface{} instead of string

[Areson]

Currently the Arg struct for spec arguments parses argument values in the spec as string types. This causes some pain for jobs that are running using values directly from the spec as they cannot be defined using more meaningful types (e.g. int or bool) and instead must expect values to be strings. Changing this to use interface{} allows values to be parsed by the yaml parser into primitives based on the content of the yaml file rather than forcing everything to be strings.

[daniel-nichter]

Talked with @Areson and this should be ok in Spin Cycle itself since the Job method is already Create(jobArgs map[string]interface{}). However, it could be a breaking change in users' jobs if a job presumes string types. Now jobs need to "dereference" the interface{} to its real underlying type. So I suggest a new minor version for this change.