message: Change update link to github.com
diff: namespace {
  const QString kTemplateFmt = "%1-XXXXXX.%2";
  -const QString kLinkFmt = "https://gitahead.com/downloads/v%1/GitAhead%2-%3.%4";
  +const QString kLinkFmt =
  + "https://github.com/gitahead/gitahead/releases/download/v%1/GitAhead%2-%3.%4";
  const QString kChangelogUrl =
  "https://raw.githubusercontent.com/gitahead/gitahead/master/doc/changelog.md";
  