require versions in package defs (#1546)

[beckjake]

Fixed #1546

seems better?:

$ dbt deps
Running with dbt=0.14.0-a1
Encountered an error:
package dependency fishtown-analytics/dbt_utils is missing a "version" field

[drewbanin]

@beckjake is there any way for us to give this error message some love? It's actually less clear from this output that the issue is a missing version. At least KeyError: version showed the missing key :)

[beckjake]

I mean, we can remove it from required and manually change it, but jsonschema kind of sucks here. There's only one valid schema!

[drewbanin]

Looks good -- approved!