[Website] Move merge_pr.py to scripts/ (#31)

We don't publish files under scripts/. merge_pr.py is an internal
script. It should not be published.
diff --git a/_config.yml b/_config.yml
index 684edd8..15cdd4d 100644
--- a/_config.yml
+++ b/_config.yml
@@ -30,12 +30,9 @@
 exclude:
   - Gemfile
   - Gemfile.lock
-  - ruby
-  - asf-site
   - scripts
   - README.md
   - vendor
-  - build-and-deploy.sh
   - themes
 
 collections:
diff --git a/merge_pr.py b/scripts/merge_pr.py
old mode 100644
new mode 100755
similarity index 99%
rename from merge_pr.py
rename to scripts/merge_pr.py
index ca4fb3c..34f7bc1
--- a/merge_pr.py
+++ b/scripts/merge_pr.py
@@ -53,7 +53,7 @@
     print("Exiting without trying to close the associated JIRA.")
     sys.exit(1)
 
-ARROW_HOME = os.path.abspath(os.path.dirname(__file__))
+ARROW_HOME = os.path.dirname(os.path.abspath(os.path.dirname(__file__)))
 PROJECT_NAME = os.environ.get('PROJECT_NAME') or 'arrow-site'
 
 # Remote name which points to the GitHub site