Stop Travis dependency cache from being invalidated unnecessarily

Storing an updated build cache for this project takes around 2 minutes,
so we want to avoid any unnecessary cache-invalidation caused by spurious
file-changes.

Examples:

1. the timestamp being touched on the empty sbt .lock files:

    change detected: /home/travis/.sbt/boot/scala-2.10.4/org.scala-sbt/sbt/0.13.7/sbt.components.lock

2. version-ranged artifacts in Ivy (pure evil)

https://twitter.com/rtyley/status/545530331672371200

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