Change the Ubuntu distribution for Travis CI

By default, Travis CI builds and run the tests on Ubuntu 14.04 LTS
(Trusty Tahr), which has OpenSSL 1.0.1f. Unfortunately, This version of
OpenSSL does not support TLS-ALPN, which has been added in 1.0.2. In
order for the tests to run, it is required to upgrade at least to Ubuntu
16.04 LTS (Xenial Xerus).

https://docs.travis-ci.com/user/reference/overview/
https://docs.travis-ci.com/user/reference/xenial/
https://packages.ubuntu.com/xenial/openssl

.travis.yml

@@ -1,4 +1,5 @@
 language: rust
+dist: xenial
 
 rust:
     - "1.32.0"