mark the dependency to scala-compiler as provided

We've got a [report](https://groups.google.com/forum/#!topic/parboiled-user/78UkvcNkeAg) that parboiled2 fetches in all of the scala-compiler and it turned out that it was a transitive dependency from shapeless2:

```
[info] default:test-pb2_2.10:0.1-SNAPSHOT [S]
[info]   +-org.parboiled:parboiled_2.10:2.0-M2 [S]
[info]     +-com.chuusai:shapeless_2.10.3:2.0.0-M1 [S]
[info]     | +-org.scala-lang:scala-compiler:2.10.3 [S]
[info]     |   +-org.scala-lang:scala-reflect:2.10.3 [S]
[info]     |   
[info]     +-org.scala-lang:scala-reflect:2.10.3 [S]
[info]     
```

It seems the dependency is only needed during compilation and only to "scala-reflect".