Only require changelog under certain conditions, such as number of files changed

[dblock]

Number of files changed, trivial commits, etc.

From https://github.com/artsy/eigen

# Oi, CHANGELOGs please
fail("No CHANGELOG changes made") if git.lines_of_code > 50 && !github.modified_files.include?("CHANGELOG.yml") && !declared_trivial