Issue Type: Bug

GitHub Enterprise 2.15.3

1. Open a workspace with a GHE repo. (In case it's relevant, the local repo has a git@<host>:<user>/<repo>.git style remote.)
2. Open the output panel
3. View Github Pull Request output

I get:

[Info] Looking for git repository
[Info] Git repository found, initializing review manager and pr tree view.
[Info] Found GitHub remote
[Info] https://<redacted>/api/v3: GraphQL not supported (Network error: Response not successful: Received status code 404)
[Info] Review> no matching pull request metadata found for current branch master

I expected graphql to work. Clicking through the API doc links on our instance of GHE brings me to this page https://developer.github.com/enterprise/2.15/v4/guides/intro-to-graphql/. Following that documentation I found that the following command works using the same token that I have configured for the vscode-github-pr extention.

curl -H "Authorization: bearer <redacted>" https://<redacted>/api/graphql

The following also works, but returns a much smaller JSON response with a series of URL patterns that doesn't look like a GraphQL result.

curl -H "Authorization: bearer <redacted>" https://<redacted>/api/v3

The extension does not enable the batch review mode feature because it cannot find the GraphQL support.

Extension version: 0.4.0
VS Code version: Code 1.31.0 (7c66f58312b48ed8ca4e387ebd9ffe9605332caa, 2019-02-05T22:32:14.164Z)
OS version: Darwin x64 17.7.0