Revert update to bionic.

This has new enough kernel headers that libev builds the io_uring
backend, but testing it fails:

  ======================================================================
220  ERROR: test_linux_iouring_0 (gevent.tests.test__close_backend_fd.Test)
221  ----------------------------------------------------------------------
222  Traceback (most recent call last):
223    File "/home/travis/.runtimes/snakepit/2.7.d/lib/python2.7/site-packages/gevent/tests/test__close_backend_fd.py", line 77, in test
224      self._check_backend(backend)
225    File "/home/travis/.runtimes/snakepit/2.7.d/lib/python2.7/site-packages/gevent/tests/test__close_backend_fd.py", line 45, in _check_backend
226      hub = Hub(backend, default=False)
227    File "/home/travis/.runtimes/snakepit/2.7.d/lib/python2.7/site-packages/gevent/hub.py", line 441, in __init__
228      self.loop = self.loop_class(flags=loop, default=default) # pylint:disable=not-callable
229    File "src/gevent/libev/corecext.pyx", line 444, in gevent.libev.corecext.loop.__cinit__
230      raise SystemError("ev_loop_new(%s) failed" % (c_flags, ))
231  SystemError: ev_loop_new(50331776) failed

There are also different SSL errors (of course) on different versions.
gevent.tests.test__makefile_ref.TestSSL commonly fails to find
expected open files on most versions, and some versions don't see the
cipher suites they expect, etc.

.travis.yml

@@ -7,7 +7,7 @@
 # python: one day so making the migration simpler). "shell" and
 # "minimal" are aliases.
 language: shell
-dist: bionic
+dist: xenial
 group: travis_latest
 os:
   - linux

deps/README.rst

@@ -62,7 +62,7 @@ Updating libuv
 - Clean up the libuv tree, and apply the patches to libuv (this whole
   sequence is meant to be copied and pasted into the terminal)::
 
-    export LIBUV_VER=v1.36.0
+    export LIBUV_VER=v1.38.0
 
     cd deps/
     wget https://dist.libuv.org/dist/$LIBUV_VER/libuv-$LIBUV_VER.tar.gz
@@ -84,4 +84,10 @@ and the build process. Evaluate those and add them to git and to
 are changes to the build system (e.g., the .gyp files) that need to be
 accounted for in our build file.
 
+.. caution::
+
+   Pay special attention to the m4 directory. New .m4 files that need
+   to be added may not actually show up in git output. See
+   https://github.com/libuv/libuv/issues/2862
+
 - Follow the same 'config.guess' and 'config.sub' steps as libev.

docs/changes/1621.feature

 Update bundled libuv from 1.36.0 to 1.38.0.
-
-Travis CI now uses Ubuntu 18.04, an upgrade from Ubuntu 16.04.