Fix eviction warnings when using sbt >= 0.13.6

We are using sbt version 0.13.6 in our project and saw a lot of warnings without being able to determine the reason:

```
[warn] There may be incompatibilities among your library dependencies.
[warn] Here are some of the libraries that were evicted:
[warn] 	* com.google.guava:guava:16.0.1 -> 17.0 
```

Turns out Guice 4.0-beta5 still depends on Guava 16.0.1. Since scala-guice compiles and tests just fine against 16.0.1 I don't see why it should need Guava 17.0. Hence I propose to downgrade the Guava version to the one depended on by Guice.

What do you think?