Update activity.go

[retgits]

Adding required field ref to the activity.json template. This would fix https://github.com/TIBCOSoftware/flogo/issues/380

What kind of change does this PR introduce? (check one with "x")

[X] Bugfix
[] Feature
[] Code style update (formatting, local variables)
[] Refactoring (no functional changes, no api changes)
[] Other... Please describe:

Current behavior (how does the issue manifest): The template for activity.json and trigger.json in the flogogen command misses the required field ref

Expected behavior: flogogen should generate correct activities

Minimal steps to reproduce the problem (not required if feature enhancement):

• flogogen activity HelloWorld
• cat activity.json <-- output misses the ref field

New behavior flogogen generates all required fields