Install kcov via APT

This prevents cargo coveralls from downloading it and compiling it on
the fly.

Author: benesch

.travis.yml

@@ -5,13 +5,19 @@ language: rust
 addons:
   apt:
     packages:
+      - kcov
       - libcurl4-openssl-dev
       - libelf-dev
       - libdw-dev
       - binutils-dev
       - cmake
     sources:
       - kalakris-cmake
+      # The version of kcov shipped with Xenial (v25) doesn't support the
+      # --verify option that `cargo coveralls` passes. This PPA has a more
+      # up-to-date version. It can be removed if Ubuntu ever ships a newer
+      # version, or replaced with another PPA if this one falls out of date.
+      - sourceline: ppa:sivakov512/kcov
 
 rust:
   - stable