Fixed local_data::Key issue

 - On the latest nightly, the build was failing because of deprecated Key instantiation. 

```
   Compiling colorize v0.0.1 (https://github.com/jeremyletang/colorize.git#ref=ab5f038c641230ea241b6d2410062e8b22693f6d)
Could not execute process `rustc src/colorize.rs --crate-name colorize --crate-type rlib --crate-type dylib -g -C metadata=colorize:-:0.0.1:-:https://github.com/jeremyletang/colorize.git#ref=ab5f038c641230ea241b6d2410062e8b22693f6d -C extra-filename=8c2764ab94add3b2 --out-dir /home/travis/build/rgawdzik/cs246test/target/deps -L /home/travis/build/rgawdzik/cs246test/target/deps -L /home/travis/build/rgawdzik/cs246test/target/deps` (status=101)
--- stderr
src/colorize.rs:138:56: 138:71 error: unresolved name `local_data::Key`.
src/colorize.rs:138     static glob_color: local_data::Key<GlobalColor> = &local_data::Key;
                                                                           ^~~~~~~~~~~~~~~
error: aborting due to previous error
```