URL encoded %2F character

I hadn't noticed this until today, which may show my lack of attention lately, but is there any reason we're URL encoding the `/` into `%2F` in Shields.io URLs like this one:

```
http://shields.io/travis-ci/rails%2Frails.png
```

Seems clear we can't replace the `/` with another character like `_` as I'm guessing we're doing this to keep the whole `org/repo` combo inside of the image's filename.

@whit537 is there a way we could avoid doing that to make the URLs more transparent/legible? This is not a make or break issue at all, I'm just curious.