Allow Github token authorization for all API versions

The current master branch allows authorization via the GITHUB_TOKEN
option only for v1 and v2. The dev branch contains a fix for this,

eduardoboucas@1f31ac8

but the pull request has been open for quite some time,

eduardoboucas#405

Unfortunately, the dev branch does not seem to work out of the box
for me, so I will just use this small patch to make Staticman work
for me for the time being.

lib/GitHub.js

@@ -18,8 +18,7 @@ class GitHub extends GitService {
     return (async () => {
       const isAppAuth = config.get('githubAppID') &&
         config.get('githubPrivateKey')
-      const isLegacyAuth = config.get('githubToken') &&
-        ['1', '2'].includes(options.version)
+      const isLegacyAuth = config.get('githubToken')
 
       let authToken