Remove the --pre pip option by default

[pytoxbot]

• Bitbucket: https://bitbucket.org/hpk42/tox/issue/193
• Originally reported by: @berdario
• Originally created at: 2014-09-28T18:43:05.125

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

#!text

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

[pytoxbot]

Original comment by @hpk42

tox-1.9 does not use --pre by default anymore.

[pytoxbot]

Original comment by @cjw296

I'll second the voices in Ned's posting: please remove --pre, it creates unpleasant breakage...

[pytoxbot]

Original comment by @dstufft

Sort of? pip install foobar>=1.4.0.dev1 would enable --pre only for foobar, not for the dependency foobar has on other-thing>=1.0 (unless of course the dependency was other-thing>=1.0.dev1.

[pytoxbot]

Original comment by @hpk42

if i do pip install >=1.4.0.dev1 does it effectively mean pip install --pre >=1.4.0.dev1?

[pytoxbot]

Original comment by @dstufft

FWIW I don't think pip ever mandated that you have --pre in order to install a pre-release, you could specify a version specifier that includes a pre-release and that would trigger it as well (see: pypa/pip#834).