Bump Aspen version to pick up a bugfix

https://github.com/gittip/aspen-python/issues/207

This is the only difference between 0.25.1 and 0.25.2.

We need this for pjax on #1184.
Merge pull request #1210 from gittip/aspen-bump

Bump Aspen version to pick up a bugfix
Fix bug in account merge (#1226)

Bountysource uses 'display_name' but we, under the influence of Twitter,
were trying to access 'screen_name'. This only shows up if there is a
bountysource account in the mix when merging accounts.
Merge branch 'master' into fix-account-merge