Use python from virtualenv

We're using an old version of virtualenv (1.7.1.2) that doesn't include
a python2.7 in bin. The result is that our release script to date has
used whatever python2.7 was available on PATH. This is problematic
because we need to have wheel installed for the python we're using to
release. Best to use the virtualenv python for the release.

We should add a check for whether we're actually in the virtualenv.
Maybe if $PS1 ends with "[aspen] "?

We should probably also upgrade virtualenv.
Go with just `python` in release.sh

This is more flexible than calling python2.7. We don't want to start
overengineering the release process by adding heavy checks for
virtualenv or python version.
Merge pull request #273 from gittip/fix-release

Use python from virtualenv