Use of yellow output makes important messages unreadable

[barry-scott]

pip3 (3.5 on Mac OS X) is outputting a message in yellow that I can barely see on a white background terminal.

"You are using pip version 7.1.2, however version 8.0.2 is available.
You should consider upgrading via the 'pip install --upgrade pip' command."

I suggest that pip only uses text output. But if the pip developers insist on using colour then set both foreground and background colours.

[xavfernandez]

Somewhat related to #2449

[dstufft]

Closing this as a duplicate of #2449.