Update cURL to 7.48.0 (#851)
Update cURL to 7.48.0 (#851)

SHA-256 taken from 7zip;
[curl manifest file tested on external bucket](http://puu.sh/oQq83/d131b4a4fa.png)

Mirror changes:
- mirror confusedbycode.com looks abandoned;
- mirror bintray.com from list Win32/Win64 - Generic (Viktor Szakáts) was chosen
- build project page for windows is hosted on https://github.com/vszakats/harbour-deps
toxeus/scoop,coonce/scoop,Cyianor/scoop,berwyn/scoop,rodericktech/scoop,nueko/scoop,reelsense/scoop,systemexitzero/scoop,rasa/scoop,bpollack/scoop,vidarkongsli/scoop,kodybrown/scoop,nikolasd/scoop,vinaynambiar/lukesampson-,Congee/scoop,yunspace/scoop,r15ch13/scoop,lukesampson/scoop,rmyorston/scoop