Travis CI: upgrade to Ubuntu 18.04

If "dist" is not specified, Travis uses Ubuntu 16.04
with kernel 4.15.0-1028-gcp. This kernel misses fixes
to FUSE which cause hangs in TestParallelDiropsHang.

Specifying "dist: bionic" makes Travis use kernel
5.0.0-1026-gcp.

Fixes https://github.com/hanwen/go-fuse/issues/281
Change-Id: Ic0f9987726eb11c608ec0cf42b73e089ec1cd2db

Change-Id: If158380a909a6bf78212c75c8dddc57e8c955448

.travis.yml

 sudo: required
+# Ubuntu 18.04 "Bionic", https://docs.travis-ci.com/user/reference/bionic/
+# Kernel 5.0.0-1026-gcp
+dist: bionic
 
 language: go
 go_import_path: github.com/hanwen/go-fuse