Multiple h1s in Changelog - there should only be one

The HTML output in the change logs produced by this tool include too many h1 elements. There should be one at the top for “Change Log”, and the individual releases should be h2’s so the content hierarchy makes sense and follows best practices for accessible headings.

Here’s what it outputs now:

# Change Log

All notable changes to this project will be documented in this file. See [standard-version](https://github.com/conventional-changelog/standard-version) for commit guidelines.

<a name="3.0.0-alpha.2"></a>
# [3.0.0-alpha.2](https://github.com/dequelabs/axe-core/compare/v3.0.0-alpha.1...v3.0.0-alpha.2) (2017-09-01)

### Bug Fixes

Here’s what it should be:

# Change Log

All notable changes to this project will be documented in this file. See [standard-version](https://github.com/conventional-changelog/standard-version) for commit guidelines.

<a name="3.0.0-alpha.2"></a>
## [3.0.0-alpha.2](https://github.com/dequelabs/axe-core/compare/v3.0.0-alpha.1...v3.0.0-alpha.2) (2017-09-01)

### Bug Fixes

I couldn’t figure out where those come from after a quick look through the source, perhaps it’s from one of the dependent modules? I’m happy to submit a PR if someone can point me in the right direction (including to another repo, if needed).

[bcoe]

This should now be fixed as of standard-version@6.x.x thanks for reporting the issue @marcysutton 👍

[aaronjameslang]

So we need to bump the version of conventional-changelog standard-version depends on. Should we update all the deps while we’re at it? Looks like it might require a major version bump, would there be resistance to this?