Cap `bytestring' dependency

For some reason, `bytestring' versions above 0.9.2.1 causes (at least)
GHC 7.0.1 to panic, so I capped the version while investigating the
issue.

pontarius-xmpp.cabal

@@ -32,7 +32,7 @@ Library
                , base              >4 && <5
                , base64-bytestring >=0.1.0.0
                , binary            >=0.4.1
-               , bytestring        >=0.9.1.9
+               , bytestring        >=0.9.1.9 && <=0.9.2.1
                , conduit           >=0.5
                , containers        >=0.5.0.0
                , crypto-api        >=0.9