Hanging on 'Downloading library build "*" (might take a while)'

On the current master branch of `mbed-cli`:
```
mbed new simple_test --mbedlib
[mbed] Working path "/home/arvoelke/git/..." (program)
[mbed] Creating new program "simple_test" (git)
[mbed] Adding library "mbed" from "https://mbed.org/users/mbed_official/code/mbed/builds" at branch/tag "tip"
[mbed] Downloading library build "65be27845400" (might take a while)
```

I've tried this three times now, each time keeping it open for more than 24 hours -- and it still hangs.

If I visit the above url (https://mbed.org/users/mbed_official/code/mbed/builds) and click on the first build (`65be27845400`), it takes me to an invalid page:

https://os.mbed.com/users/mbed_official/code/mbed//builds/65be27845400/
> An error occurred while processing your request
> no such method: builds

Note there are two slashes (`//`) in the URL. If I remove one of the slashes then the link becomes valid. Is there a fix or work-around? Thanks.