xsel: 1.2.0 -> git-2016-09-02

The last xsel release was in 2008. Since then, a number of bugs have
been fixed. For example, pasting a large chunk of text (>4000 chars)
into chrome would cause the current tab to hang indefinitely.

A couple examples in the wild:

https://github.com/kfish/xsel/issues/14
https://github.com/kfish/xsel/issues/13
https://github.com/kfish/xsel/pull/16
https://bugs.chromium.org/p/chromium/issues/detail?id=515401
https://github.com/electron/electron/issues/3116
https://github.com/neovim/neovim/issues/4719