Fix implicit conversion method Scala compiler warnings

[jbaiera]

Subtask of #969

Just need to import the correct language features (scala.language.implicitConversions) on the files in the Spark connector that are affected.

/Users/james.baiera/Documents/source/elasticsearch-hadoop/spark/sql-20/src/main/scala/org/elasticsearch/spark/streaming/package.scala:31: implicit conversion method sparkDStreamFunctions should be enabled
by making the implicit value scala.language.implicitConversions visible.
This can be achieved by adding the import clause 'import scala.language.implicitConversions'
or by setting the compiler option -language:implicitConversions.
See the Scala docs for value scala.language.implicitConversions for a discussion
why the feature should be explicitly enabled.
  implicit def sparkDStreamFunctions(ds: DStream[_]): SparkDStreamFunctions = new SparkDStreamFunctions(ds)
               ^
/Users/james.baiera/Documents/source/elasticsearch-hadoop/spark/sql-20/src/main/scala/org/elasticsearch/spark/streaming/package.scala:39: implicit conversion method sparkStringJsonDStreamFunctions should be enabled
by making the implicit value scala.language.implicitConversions visible.
  implicit def sparkStringJsonDStreamFunctions(ds: DStream[String]): SparkJsonDStreamFunctions[String] = new SparkJsonDStreamFunctions[String](ds)
               ^
/Users/james.baiera/Documents/source/elasticsearch-hadoop/spark/sql-20/src/main/scala/org/elasticsearch/spark/streaming/package.scala:40: implicit conversion method sparkByteArrayJsonDStreamFunctions should be enabled
by making the implicit value scala.language.implicitConversions visible.
  implicit def sparkByteArrayJsonDStreamFunctions(ds: DStream[Array[Byte]]): SparkJsonDStreamFunctions[Array[Byte]] = new SparkJsonDStreamFunctions[Array[Byte]](ds)
               ^
/Users/james.baiera/Documents/source/elasticsearch-hadoop/spark/sql-20/src/main/scala/org/elasticsearch/spark/streaming/package.scala:48: implicit conversion method sparkPairDStreamFunctions should be enabled
by making the implicit value scala.language.implicitConversions visible.
  implicit def sparkPairDStreamFunctions[K : ClassTag, V : ClassTag](ds: DStream[(K,V)]): SparkPairDStreamFunctions[K,V] = new SparkPairDStreamFunctions[K,V](ds)
               ^