_apt-get --yes install_ is better for non-interactive installs

[jdjohnston]

As the line is currently written, installing Gtk.jl failed for me because I didn't have libgtk-3-dev and I wasn't given an opportunity to answer yes when apt-get prompted for permission to continue the install.

An easy fix would be to insert --yes between apt-get and install on line 454 of dependencies.jl. The --yes option is, in fact, intended for non-interactive installs.