Minor cleanup of semicolons and use Vector.newBuilder for improved performance

```scala
Welcome to Scala version 2.9.3 (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_41).
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scala> import com.twitter.util.Time
import com.twitter.util.Time

scala> import scala.collection._
import scala.collection._

scala> var vector = immutable.Vector.empty[Int]
vector: scala.collection.immutable.Vector[Int] = Vector()

scala> println(Time.measure { (0 to 50000000).foreach { vector :+= _ } }.inMillis)
7395

scala> val b = Vector.newBuilder[Int]
b: scala.collection.mutable.Builder[Int,scala.collection.immutable.Vector[Int]] = scala.collection.immutable.VectorBuilder@b06cf66

scala> println(Time.measure { (0 to 50000000).foreach { b += _ }; b.result }.inMillis)
2855

scala>
```