Stop Travis dependency cache from being invalidated unnecessarily

Storing an updated build cache for this project takes around 2 minutes,
and is triggered unnecessarily by the timestamp being touched on the empty
sbt .lock files:

https://travis-ci.org/guardian/gu-who/builds/44429369#L80
change detected: /home/travis/.sbt/boot/scala-2.10.4/org.scala-sbt/sbt/0.13.7/sbt.components.lock

http://docs.travis-ci.com/user/build-configuration/#script
http://docs.travis-ci.com/user/languages/scala/#Default-Test-Command