build(gocd): Bump gocd-jsonnet to v3.0.7 (#579)

Bumps the [gocd-jsonnet](https://github.com/getsentry/gocd-jsonnet)
library used to render the GoCD deployment pipelines from `v3.0.4` to
`v3.0.7`.

- Updated `gocd/templates/jsonnetfile.json` to `v3.0.7`
- Refreshed `gocd/templates/jsonnetfile.lock.json` via `jb update`
(commit `5eac1ff`, new sum)

Vendored library files were unchanged between the two tags.

Author: dmajere

gocd/templates/jsonnetfile.json

@@ -8,7 +8,7 @@
           "subdir": "libs"
         }
       },
-      "version": "v3.0.4"
+      "version": "v3.0.7"
     }
   ],
   "legacyImports": true

gocd/templates/jsonnetfile.lock.json

@@ -8,8 +8,8 @@
           "subdir": "libs"
         }
       },
-      "version": "7a25c44956933a57b5ca03beb3b70c470724c2c8",
-      "sum": "0S+eGE0WHdxJxKS/Bdga+NB+cB+ZBwF4LoehKv0azXc="
+      "version": "5eac1ffce92df58ea41712ad8c82f55aa628350f",
+      "sum": "V0sY265XbQP5OVTjn8s/ZQdyRQHzndWNJs+wY1zgr8M="
     }
   ],
   "legacyImports": false