feat(spec): version field (#85)

Adds a `version` field to the `jsonnetfile.json`, so that `jb` can automatically recognize too old / too new schema versions, instead of panicking.

.drone.yml

@@ -35,8 +35,6 @@ steps:
     event:
       exclude:
       - tag
-  depends_on:
-  - gomod
 
 - name: build-1.12
   pull: always
@@ -52,8 +50,6 @@ steps:
     event:
       exclude:
       - tag
-  depends_on:
-  - gomod
 
 - name: build-1.13
   pull: always
@@ -69,8 +65,6 @@ steps:
     event:
       exclude:
       - tag
-  depends_on:
-  - gomod
 
 - name: build-1.14-rc
   pull: always
@@ -86,10 +80,6 @@ steps:
     event:
       exclude:
       - tag
-  depends_on:
-  - build-1.11
-  - build-1.12
-  - build-1.13
 
 - name: generate
   pull: always
@@ -105,7 +95,5 @@ steps:
     event:
       exclude:
       - tag
-  depends_on:
-  - build-1.13
 
 ...