travis: use trusty for coverage build

This works now and precise is in the process of being decommissioned.

Closes https://github.com/curl/curl/pull/2476

.travis.yml

@@ -85,7 +85,7 @@ matrix:
           env: T=cmake
         - os: linux
           compiler: gcc
-          dist: precise
+          dist: trusty
           env: T=coverage
         - os: linux
           compiler: gcc