Fix travis badge in README

After moving the repository under `rust-osdev`, we need to update the pointer to the Travis build status

Author: robert-w-gries

README.md

@@ -1,5 +1,5 @@
 # multiboot2-elf64
-[![Build Status](https://travis-ci.org/phil-opp/multiboot2-elf64.svg?branch=master)](https://travis-ci.org/phil-opp/multiboot2-elf64)
+[![Build Status](https://travis-ci.org/rust-osdev/multiboot2-elf64.svg?branch=master)](https://travis-ci.org/rust-osdev/multiboot2-elf64)
 
 An experimental Multiboot 2 crate for ELF-64 kernels. It's still incomplete, so please open an issue if you're missing some functionality. Contributions welcome!