Clean up bluejeans PKGBUILD

* Properly quote variables.
* Use `sha256sums` instead of `sha1sums` and `md5sums`.
* Remove "BlueJeans" from package description.
* Fiddle with quotes. Prefer single quotes over double quotes, remove quotes
  entirely for simple strings (e.g. `license`), and add quotes to potentially
  troublesome strings (e.g. `url`).