Allow either wxPython 2.8 or 3.0 for Debian package

The current testing version of Debian only includes wxPython 3.0.
Since the name of the Debian package includes the version number this
means that the Debian package needs to allow 3.0 as well as 2.8 to
support both current and future Debian and Debian derived Linux distributions.

Thanks to @cboettig for pointing this out.

Fixes #247.
Merge pull request #248 from ethanwhite/wx-version

Allow either wxPython 2.8 or 3.0 for Debian package