[btcsuite/btcutil] f3db5c: Use container-based builds on TravisCI.

[Dave Collins]

  Branch: refs/heads/master
  Home:   https://github.com/btcsuite/btcutil
  Commit: f3db5cf7e5f579e97b8cb9a4f19d52bbe6ce1712
      
https://github.com/btcsuite/btcutil/commit/f3db5cf7e5f579e97b8cb9a4f19d52bbe6ce1712
  Author: Dave Collins <da...@conformal.com>
  Date:   2015-07-28 (Tue, 28 Jul 2015)

  Changed paths:
    M .travis.yml
    A goclean.sh

  Log Message:
  -----------
  Use container-based builds on TravisCI.

Also, import the goclean.sh script directly into this repository and
call it directly instead of using a remote gist.