Travis: Add mysql to the list of services

Ubuntu Xenial, which is now being rolled out as the default Linux distro
for Travis builds following the end of life of Ubuntu Trusty, does not
start services such as MySQL unless told to. We use the default distro
in our builds so we are likely to be affected.

.travis.yml

@@ -3,6 +3,9 @@ language: perl
 os:
   - linux
 
+services:
+  - mysql
+
 perl:
   - '5.14'
   - '5.26'