Change license field per Kurt Hornik's suggestion

From Kurt:

Now, file LICENSE starts with

Included below is license and copyright information for externally
maintained
libraries used by httpuv. All other code in httpuv is copyright RStudio,
Inc.
and is provided under the terms of the GNU General Public License
version 3.

which may be taken to mean something different from everything being
compatible with GPL as you write above.

Also, from what I can see file LICENSE does not provide any
*restrictions* to the GPL-3 base license: so I guess

   License: GPL-3 | file LICENSE

   would be more appropriate.