Fix broken links pointing to GitHub

It seems like 2ec20ce228b removed a trailing slash from the `less.master`
template variable. This broke many links, like ones to the changelog:

```md
Released [v1.7.0]({{ less.master }}CHANGELOG.md)
```

which 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 fix those links, and undoes a couple
of spot changes that others made to fix individual links (changes that added a
slash in the template instead of to the template variable).