Tried miniboxing for the example

It's not yet as fast as the `DoubleVec` implementation and it does
require a `ClassTag` :(

Here are the numbers for miniboxing:
```
[info] Benchmark                                       Mode  Samples        Score  Score error  Units
[info] r.VecMapBenchmark.squareDoubleArrayWithLoop    thrpt       20  1505760.754    43246.923  ops/s
[info] r.VecMapBenchmark.squareDoubleArrayWithMap     thrpt       20    70609.607     3936.651  ops/s
[info] r.VecMapBenchmark.squareDoubleVec              thrpt       20   547807.506     9487.612  ops/s
```
Both points should be addressed when I fix https://github.com/miniboxing/miniboxing-plugin/issues/81

Ping @tixxit: I will wait until I have better numbers for a pull request :)

Just for reference, the original blog post: http://io.pellucid.com/blog/optimistic-respecialization