Error if system GUI is disabled

[Mrfjz]

omxplayer will throw error if system GUI is disabled.

omxplayer test.mp3 -o local --blank --no-osd

omxplayer.bin: omxplayer.cpp:473: void blank_background(uint32_t): Assertion `display' failed.
/usr/bin/omxplayer: line 67: 3170 Aborted > LD_LIBRARY_PATH="$OMXPLAYER_LIBS${LD_LIBRARY_PATH:+:$LD_LIBRARY_PATH}" $OMXPLAYER_BIN "$@"

Perhaps we should remove "--blank" from https://github.com/Ap0c/node-omxplayer/blob/master/index.js#L38 ?