usbdevice/core/core.c: fix garbage value bug

This could crash the program with -O1 and -O2 in MDK.
Merge pull request #81 from grissiom/fix-usb

usbdevice/core/core.c: fix garbage value bug
Update README.md
Update .travis.yml
add RAMFS implementation
Merge branch 'master' of https://github.com/RT-Thread/rt-thread