macOS's grep is not compatible gnu grep

macOS's grep (BSD grep 2.5.1-FreeBSD) is not compatible gnu grep.
So, commit-msg hook is always ERROR.

```
git secrets --commit_msg_hook -- foo.txt
test1.txt:1:test1

[ERROR] Matched one or more prohibited patterns

Possible mitigations:
- Mark false positives as allowed using: git config --add secrets.allowed ...
- Mark false positives as allowed by adding regular expressions to .gitallowed at repository's root directory
- List your configured patterns: git config --get-all secrets.patterns
- List your configured allowed patterns: git config --get-all secrets.allowed
- List your configured allowed patterns in .gitallowed at repository's root directory
- Use --no-verify if this is a one-time false positive
```