nix: write to ~/.curlrc
nix: write to ~/.curlrc

Nix wasn't respecting the "silent" argument all of the time. This should
disable the "progress" bar updating style through the .curlrc config.

JuliaCI/travis-build,andyli/travis-build,JuliaCI/travis-build,andyli/travis-build,kevmoo/travis-build,craigcitro/travis-build,kevmoo/travis-build,craigcitro/travis-build