Update the Travis CI configuration

The `dist: xenial` has been added because, at the time, build was done
using an old version of Ubuntu including an unsupported version of
OpenSSL. Now that Xenial is the default build version, this option
should be removed.
See commit 261e0e50fd

.travis.yml

@@ -1,5 +1,4 @@
 language: rust
-dist: xenial
 
 rust:
     - "1.40.0"