Fix merge script issues

Problem:

Currently the merge script is broken when the reviewer's id is not in reviewers list.

Solution:

- Change "reviewers[reviewer_id]" to "reviewer_id in reviewers"
- Bump the default version to 0.5.0.

Author: Sijie Guo <sijie@apache.org>

Reviewers: Jia Zhai <None>

Closes #134 from sijie/fix_merge_script_issues
diff --git a/scripts/dev/dl-merge-pr.py b/scripts/dev/dl-merge-pr.py
index 6c9d3e8..a669fd4 100755
--- a/scripts/dev/dl-merge-pr.py
+++ b/scripts/dev/dl-merge-pr.py
@@ -84,7 +84,7 @@
 RELEASE_BRANCH_PREFIX = ''
 
 DEV_BRANCH_NAME = 'master'
-DEFAULT_FIX_VERSION = os.environ.get("DEFAULT_FIX_VERSION", "0.4.0")
+DEFAULT_FIX_VERSION = os.environ.get("DEFAULT_FIX_VERSION", "0.5.0")
 
 def get_json(url, preview_api = False):
   """
@@ -450,7 +450,7 @@
   for reviewer_id in reviewers_ids:
     username = None
     useremail = None
-    if reviewers[reviewer_id] is not None:
+    if reviewer_id in reviewers:
       reviewer = reviewers[reviewer_id]
       username = reviewer['name']
       useremail = reviewer['email']