[Pytest-commit] Issue #193: Remove the --pre pip option by default (hpk42/tox)

[Dario Bertini]

New issue 193: Remove the --pre pip option by default
https://bitbucket.org/hpk42/tox/issue/193/remove-the-pre-pip-option-by-default

Dario Bertini:

This can [break some builds](http://nedbatchelder.com/blog/201409/how_should_i_distribute_coveragepy_alphas.html#commentform)

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