This can break some builds
I looked at the Changelog for when this change was introduced, and apparently pip would refuse to install the exactly specified prerelease dependencies unless the
--pre flag was supplied
Also the default [py25] environment modifies the default installer_command (new config option) to use pip without the "--pre" which was introduced with pip-1.4 and is required if you want to install non-stable releases. (tox defaults to install with "--pre" otherwise).
This is understandable, but apparently it's not the case anymore, since if you run
pip install coverage==4.0a1 it will install the package without making a fuss
I don't know when pip changed its behavior, but it'd be nice to take it into account now