[SPARK-10650] Clean before building docs

The [published docs for 1.5.0](http://spark.apache.org/docs/1.5.0/api/java/org/apache/spark/streaming/) have a bunch of test classes in them.  The only way I can reproduce this is to `test:compile` before running `unidoc`.  To prevent this from happening again, I've added a clean before doc generation.

Author: Michael Armbrust <michael@databricks.com>

Closes #8787 from marmbrus/testsInDocs.

docs/_plugins/copy_api_dirs.rb

@@ -26,12 +26,15 @@ if not (ENV['SKIP_API'] == '1')
     curr_dir = pwd
     cd("..")
 
-    puts "Running 'build/sbt -Pkinesis-asl compile unidoc' from " + pwd + "; this may take a few minutes..."
-    puts `build/sbt -Pkinesis-asl compile unidoc`
+    puts "Running 'build/sbt -Pkinesis-asl clean compile unidoc' from " + pwd + "; this may take a few minutes..."
+    puts `build/sbt -Pkinesis-asl clean compile unidoc`
 
     puts "Moving back into docs dir."
     cd("docs")
 
+    puts "Removing old docs"
+    puts `rm -rf api`
+
     # Copy over the unified ScalaDoc for all projects to api/scala.
     # This directory will be copied over to _site when `jekyll` command is run.
     source = "../target/scala-2.10/unidoc"