Quotes needed around test in example SBT settings

Literally using the example build configuration in the README with SBT 0.13.5 gave me:

```
/Users/richard/Developer/typelevel-kitchensink/build.sbt:48: error: overloaded method value % with alternatives:
  (configurations: String)sbt.ModuleID <and>
  (configuration: sbt.Configuration)sbt.ModuleID
 cannot be applied to (sbt.TaskKey[Unit])
  "com.github.julien-truffaut"  %%  "monocle-law"     % monocleVersion % test
                                                                       ^
```

The fix for me was to replace `test` with `"test"`.

Re: https://github.com/julien-truffaut/Monocle/issues/89