Remove CFLAGS from .travis.yml
Add __unused definition to trek.h
Re-add CFLAGS to .travis.yml
Remove srandomdev() for Linux compatibility

* Add `#include <time.h>` in trek.h
* Change srandomdev() to srandom(time(NULL)) in main.c
main.c: force srandom seed cast to unsigned
Add Linux/Travis CI doc on README.md