Back off Travis build to default dist

Travis appears to have deployed a testing xenial environment which
runs a daemon that sometimes interferes with apt commands.  Remove the
"dist: xenial" line so we run in the default, supported distribution
(currently trusty).

.travis.yml

@@ -2,8 +2,6 @@ language: c++
 
 sudo: required
 
-dist: xenial
-
 matrix:
   include:
   - compiler: clang