Vendorise nickstenning/trie

Pull in the source of `trie`, used by `triemux`, to workaround Go's oddities
with dependency versioning. This will ensure that the Router builds against
the correct version and not against Git master/HEAD.

This comes at the small expense of not making it as easy to use as a generic
library from other projects. However if that becomes a big deal in the
future then we can address it then.

Changed import statement in `triemux` accordingly. Cut from:

https://github.com/nickstenning/trie/tree/f14091f4e64174929178166659349a59f7707909
Run trie tests from within this repo

Previously these were run from Travis. They'll now be included in the
Router's normal CI tests.
Adjust trie LICENSE and README

This was developed for GDS. So I think it's acceptable to modify the LICENSE
copyright.

Godoc link should now point to this repo.
Merge pull request #49 from alphagov/59399736-vendorise_trie

[#59399736] Vendorise nickstenning/trie