Make release notes helper script more portable

[seancolsen]

We have a find_missing_prs.sh script that helps us generate release notes. It works fine on linux but won't run on Mac OS due to some issues pointed out by @Anish9901. We should fix these so that the script run on Mac OS too.

From Anish:

Here's the specific error that I was facing on a Mac:

anish@appy releases % ./find_missing_prs.sh 0.1.6
sed: 1: "0.1.6.md": invalid command code .
date: illegal option -- d
usage: date [-jnRu] [-I[date|hours|minutes|seconds]] [-f input_fmt]
            [-r filename|seconds] [-v[+|-]val[y|m|w|d|H|M|S]]
            [[[[mm]dd]HH]MM[[cc]yy][.SS] | new_date] [+output_fmt]
grep: invalid option -- P
usage: grep [-abcdDEFGHhIiJLlMmnOopqRSsUVvwXxZz] [-A num] [-B num] [-C[num]]
  [-e pattern] [-f file] [--binary-files=value] [--color=when]
  [--context[=num]] [--directories=action] [--label] [--line-buffered]
  [--null] [pattern] [file ...]
Binder Error: Table "pr" does not have a column named "mergeCommit"
LINE 7:     ON commit.hash = pr.mergeCommit
                             ^
No missing PRs

[seancolsen]

Additional thoughts:

• Seems like there are issues with sed, date, and grep. Sheesh! I hate bash portability. But I imagine this would be a relatively quick fix for someone to make who can test on Mac OS.
• I also wouldn't be opposed to rewriting this in Python. It's quite simple.