Don't try to read version.txt when it does not exist
Don't try to read version.txt when it does not exist

This can happen for example when the arduino directory is a checkout
from git instead of a released version. Before, cat would show an error
which is now prevented. The version still defaults to 100 just like
before.

WeAreLeka/Arduino-Makefile,thomaskilian/Arduino-Makefile,kindsenior/Arduino-Makefile,AlexSatrapa/Arduino-Makefile,exmachina-dev/Arduino-Makefile,tuna-f1sh/Arduino-Makefile,marioaugustorama/Arduino-Makefile,prpplague/Userspace-Arduino,drpjk/Arduino-Makefile,sudar/Arduino-Makefile,drpjk/Arduino-Makefile,thomaskilian/Arduino-Makefile,jamesrob4/Arduino-Makefile,tuna-f1sh/Arduino-Makefile,drpjk/Arduino-Makefile,WeAreLeka/Arduino-Makefile,jamesrob4/Arduino-Makefile,theicfire/Arduino-Makefile,sudar/Arduino-Makefile,bugobliterator/BUAGI,harshavardhana/arduino-makefile,joshsamuelson/Arduino-Makefile,harshavardhana/arduino-makefile,WeAreLeka/Arduino-Makefile,sej7278/Arduino-Makefile,sej7278/Arduino-Makefile,marioaugustorama/Arduino-Makefile,prpplague/Userspace-Arduino,kindsenior/Arduino-Makefile,bugobliterator/BUAGI,prpplague/Userspace-Arduino,bugobliterator/BUAGI,exmachina-dev/Arduino-Makefile,theicfire/Arduino-Makefile,AlexSatrapa/Arduino-Makefile,bugobliterator/BUAGI,bugobliterator/BUAGI,prpplague/Userspace-Arduino,joshsamuelson/Arduino-Makefile,bugobliterator/BUAGI