Change the ⇒ character (maybe from scalariform) to => in Scala code for style consistency

Looks like there are some ⇒ Unicode character (maybe from scalariform) in Scala code.
This PR is to change it to => to get some consistency on the Scala code.

If we want to use ⇒ as default we could use sbt plugin scalariform to make sure all Scala code has ⇒ instead of =>

And remove unused imports found in TwitterInputDStream.scala while I was there =)