Fix broken links pointing to GitHub

It seems like 2ec20ce228b removed a trailing slash from the `less.master` template variable. This broke some links, like ones to the changelog:

```md
Released [v1.7.0]({{ less.master }}CHANGELOG.md)
```

which lost a slash and rendered like:

```html
<p>Released <a href="https://github.com/less/less.js/blob/masterCHANGELOG.md">v1.7.0</a></p>
```

This commit re-adds the trailing slash to `less.master` to fix those links, and undoes a couple of spot changes that others have made to fix individual links (like #202).

`less.rawmaster` also had a trailing slash added, but this was just to increase similarity with `less.master`, not to fix any broken links.