correct scalatest to run with the good classpath

Hi,

Scalatest doesn't run well test (I've got strange errors like :
```java
*** RUN ABORTED ***
  java.lang.IllegalAccessError: tried to access method com.mongodb.CommandResult.<init>(Lcom/mongodb/ServerAddress;)V from class com.mongodb.FongoDB
  at com.mongodb.FongoDB.okResult(FongoDB.java:131)
  at com.mongodb.FongoDBCollection.insertResult(FongoDBCollection.java:51)
  at com.mongodb.FongoDBCollection.insert(FongoDBCollection.java:87)
  at com.mongodb.FongoDBCollection.insert(FongoDBCollection.java:65)
  at com.mongodb.DBCollection.insert(DBCollection.java:60)
  at com.mongodb.DBCollection.insert(DBCollection.java:105)
  at com.foursquare.fongo.FongoScalaTest$$anonfun$2.apply$mcV$sp(FongoScalaTest.scala:21)
  at com.foursquare.fongo.FongoScalaTest$$anonfun$2.apply(FongoScalaTest.scala:18)
  at com.foursquare.fongo.FongoScalaTest$$anonfun$2.apply(FongoScalaTest.scala:18)
  at org.scalatest.FlatSpec$$anon$1.apply(FlatSpec.scala:3062)
```java

With fork = once, no more problem.

I update the mongodb driver, and the scalatest to go to FlatSpec instead of Suite (who is now deprecated)