Updated travis/Makefile
Updated travis/Makefile

Added soruce files to travis/Makefile:
- util/string_util.cpp
- hal/common/file.cpp
- hal/common/serial.cpp
- hal/common/dbg.cpp
- hal/linux/serial_linux_io.cpp

lis-epfl/MAVRIC_Library,lis-epfl/MAVRIC_Library,lis-epfl/MAVRIC_Library