Allow customization of MELPA repository url.

Allow customization of MELPA repository url.
I need this change to be able to use https url (it's the only way to get through proxy at work).
Does it work for you?

I think it should be:
```el
(list :url quelpa-melpa-repository-url)
```
or
```el
`(:url ,quelpa-melpa-repository-url)
```
but not
```el
'(:url quelpa-melpa-repository-url)
```