FIX [mitchellh/vagrant#4889] .curlrc ignored

change "-q" to "--no-progress-bar" in downloader.rb and downloader_test.rb

The initial fix for mitchellh/vagrant#4494 created a regression by causing curlrc to be completely ignored. By ignoring curlrc, significant curl functionality, such as proxy use, configuration of CA certs, and out-of-source password storage (via netrc), was made difficult or impossible.

This repair, suggested by @slicen, more narrowly repairs the original issue (mitchellh/vagrant#4494) and still allows for the use of curlrc configuration directives.

[1] mitchellh/vagrant#4889
[2] mitchellh/vagrant#4494