Allow pip to install pre-release versions

As of version 1.4 of pip, you have to specify a "--pre" flag to install pre-release versions. This is notably a problem for the `pytz` package, which has non-standard version numbering.

This patch adds support for "--pre" to salt.

https://github.com/pypa/pip/commit/4d5c5f8f977d55f84ac6d7ea97c213ed91ca081d