SPARK-1151: Update dev merge script to use spark.git instead of incubator-spark

Author: Thomas Graves <tgraves@apache.org>

Closes #47 from tgravescs/fix_merge_script and squashes the following commits:

8209ab1 [Thomas Graves] Update dev merge script to use spark.git instead of incubator-spark

dev/merge_spark_pr.py

@@ -38,7 +38,7 @@ PR_REMOTE_NAME = os.environ.get("PR_REMOTE_NAME", "apache-github")
 # Remote name which points to Apache git
 PUSH_REMOTE_NAME = os.environ.get("PUSH_REMOTE_NAME", "apache")
 
-GIT_API_BASE = "https://api.github.com/repos/apache/incubator-spark"
+GIT_API_BASE = "https://api.github.com/repos/apache/spark"
 # Prefix added to temporary branches
 BRANCH_PREFIX = "PR_TOOL"