Replace solarized with seoul256 as a light theme.

- seoul256 is less buggy and actively developed.  It also integrates better with
  other plugins like Airline and GitGutter.
Remove FixGitvAirlineTheme and related autocmds.

- Unnecessary now that solarized has been removed.
Remove FixGitGutterSignColumn.

- Unnecessary now that solarized has been removed.
Call AirlineRefresh with silent! in SetupColorScheme()