Improvements to update notifier

- Only report an update available if the version number of the release
  is greater than the current version

- Removes `command` dependency from `update` package; instead, pass
  current version as an argument

- Remove `brew upgrade` instructions since we can't be certain that gh
  was installed via Homebrew in the first place.

- Does not check for updates unless stderr is a tty

- Preserve stderr color output even if stdout is not a tty

- Fixes stderr color output on Windows

test/fixtures/latestRelease.json

@@ -1,4 +0,0 @@
-{
-  "tag_name": "v1.0.0",
-  "html_url": "https://www.spacejam.com/archive/spacejam/movie/jam.htm"
-}