remove bower

bower has been deprecated by its maintainers. I don't have publish
on bower anyway. So..removing bower from README and removing bower.json.
Please use npm, yarn, whatever else you might like instead.

Ref: https://bower.io/blog/2017/how-to-migrate-away-from-bower/

Author: Trott

README.md

@@ -15,10 +15,6 @@ Make strings url-safe.
 npm install slug
 ```
 
-```
-bower install slug
-```
-
 ## example
 
 ```javascript