[btcsuite/btcutil] 52bb44: Update TravisCI for Go 1.6.

[Dave Collins]

  Branch: refs/heads/master
  Home:   https://github.com/btcsuite/btcutil
  Commit: 52bb44a147d62c23a258d5c68106dbcc00b63dc8
      
https://github.com/btcsuite/btcutil/commit/52bb44a147d62c23a258d5c68106dbcc00b63dc8
  Author: Dave Collins <da...@conformal.com>
  Date:   2016-03-22 (Tue, 22 Mar 2016)

  Changed paths:
    M .travis.yml

  Log Message:
  -----------
  Update TravisCI for Go 1.6.

Now that Go 1.6 has been released, remove the old Go versions from the
configurations tested by TravisCI and add the latest point releases.