upgrade test bot to xenial (ubuntu 16.04 LTS)

Older glibc seemingly doesn't have right unwind info for signal frame,
so fails recently upgraded stacktrace_unittest. But since this version
is not supported anymore, lets just test newer glibc.

.travis.yml

@@ -1,4 +1,4 @@
 language: c++
 sudo: required
-dist: trusty
+dist: xenial
 script: ./autogen.sh && ./configure && make -j`getconf _NPROCESSORS_ONLN` && make check