Use "substring match" rather than "starts with" selector to match responsive units.

Apologies for not catching this sooner, but the responsive units matching:

```
.pure-g-r > [class ^= "pure-u"] {
    width: 100%;
}  
```

fails when another class precedes the unit class.

This PR updates the selector to the following:

```
.pure-g-r > .pure-u,
.pure-g-r > [class *= "pure-u-"] {
    width: 100%;
}
```

I've added the trailing hyphen to avoid matching on `pure-user` or other potential classes that may start with `pure-u`.

One caveat is that this will also match classes that include the string `pure-u-`, for example `impure-u-*`.