.travis.yml: Add valgrind testing

Currently, our Travis builds don't have valgrind installed, meaning
that ccanlint's valgrind based tests will be skipped, which is
unfortunate.

This adds valgrind to some of the builds to give us better CI
coverage.  It's not added for Precise with gcc, because that causes
failures which appear to be due to something in the builtins of that
gcc version.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

.travis.yml

@@ -6,7 +6,15 @@ matrix:
   - compiler: gcc
     env: CFLAGS="-std=gnu99"
   - compiler: clang
+    addons:
+      apt:
+        packages:
+          - valgrind
   - dist: trusty
+    addons:
+      apt:
+        packages:
+          - valgrind
     env: CFLAGS="-std=gnu99"
     compiler: gcc