(maint) Update commits.rake to check for "Release prep"

Currently, the "Static Code Analysis / Run checks" will fail on release prep
PRs because the check does not include "Release prep". This commit updates the
commits Rake task to check for "Release prep".

Author: AriaXLi

rakelib/commits.rake

@@ -7,7 +7,7 @@ task(:commits) do
   %x{git log --no-merges --pretty=%s #{commit_range}}.each_line do |commit_summary|
     # This regex tests for the currently supported commit summary tokens.
     # The exception tries to explain it in more full.
-    if /^\((maint|packaging|doc|docs|pa-\d+)\)|revert/i.match(commit_summary).nil?
+    if /^Release prep|\((maint|packaging|doc|docs|pa-\d+)\)|revert/i.match(commit_summary).nil?
       raise "\n\n\n\tThis commit summary didn't match CONTRIBUTING.md guidelines:\n" \
         "\n\t\t#{commit_summary}\n" \
         "\tThe commit summary (i.e. the first line of the commit message) should start with one of:\n"  \