Remove Linux distribution from Travis config.

Trusty is quite old and Xenial is the default. We should be fine not
giving any distribution. This will give us a more recent OpenSSL
version.

.travis.yml

@@ -19,8 +19,6 @@ rust:
 os:
   - linux
 
-dist: "trusty"  # we need python3-pip
-
 addons:
   apt:
     packages: