Issue title: 404 on ftpmirror.gnu.org installs
 Issue body: In trying to install wget and aspell I am getting 404 errors. It seems that ftpmirror.gnu.org is not resolving to a valid mirror url for my location (I'm in Canada). For instance my closest mirror is the CS Club in Waterloo, aspell is hosted there (I can browse to the file) but the url returned by gnu is: http://mirror.csclub.uwaterloo.ca/gnuaspell/aspell-116.69.203.115.tar.gz which is invalid. Should the mirror link be reverted back to the main GNU link (or at least have the option)?

 Comments: 
 Comment 0: I am having similar problems today as well (though I'm in the US), specifically trying to install readline. The requested URL is:

```
Error: Download failed: http://ftpmirror.gnu.org/readline/readline-6.2.tar.gz
```

When I enter this into the browser, I get a mirrored URL like:

```
http://mirror.anl.gov/pub/gnureadline/readline-6.2.tar.gz
```

It looks like the GNU FTP mirror might not be adding a / correctly. When I manually change the URL to:

```
http://mirror.anl.gov/pub/gnu/readline/readline-6.2.tar.gz
```

The file is found and downloads. 

In the meantime, is there a way to manually download these .gz files and have brew install them?

 Comment 1: Mirrors added in 164c57f284aa254a96935bb75ca5f25cf4e50714, `brew update`.

{'source': 'GithubIssues', 'repo_name_with_owner': 'Homebrew/legacy-homebrew', 'issue_id': 2399861, 'repo_stars': 27211.0}