`defaultBuild` config does not support previous builds

[trevor-bliss]

In .git2gus/config.json if the defaultBuild option is not set to the most recent build, no Gus work item is created when the Gus creation label is added.

Here is our previous config that was silently failing to create a new Gus work item when BUG P2 is added to a Github Issue:

{
  "productTag": "<omitted>",
  "defaultBuild": "222"
}

Here is the new config that successfully creates work items:

{
  "productTag": "<omitted>",
  "defaultBuild": "224"
}

Even on an older defaultBuild, work items should be created and targeted to that release.