#27 Fix build with gcc 6

[Dominique Leuenberger]

Build fails with gcc 6:

[ 42s] CdrDriver.cc:498:64: error: narrowing conversion of '255' from 'int' to 'char' inside { } [-Wnarrowing]
[ 42s] char CdrDriver::REMOTE_MSG_SYNC_[4] = { 0xff, 0x00, 0xff, 0x00 };