[Core] Modify #1869 fix to only apply to lt <= 0.15

A fix was implemented in 0.16: https://code.google.com/p/libtorrent/issues/detail?id=406#c39
[GTKUI] Fix ImportError with ReactorAlreadyInstalledError

Older systems such as Ubuntu Lucid encountered this import error as Twisted versions < 10 don't
have the exception type ReactorAlreadyInstalledError.