adds bumping for composer versions

[TilmannBach]

this fixes #261

[coveralls]

Coverage remained the same at 100.0% when pulling 0724841 on TilmannBach:patch-1 into e5c99f6 on conventional-changelog:master.

[bcoe]

@TilmannBach this is great 👍 sorry for taking so long to land.

It's on my mind right now:

1. how to better reflect that standard-version works for a variety of development environments other than Node.js.
2. I'd like to make standard-version's support of languages even broader, with reasonable default behavior for codebases with no manifest.

[bcoe]

@TilmannBach sorry this took literally forever to land, go ahead and try:

npm i standard-version@5.0.0

Appreciate the contribution; in the future, feel free to ping me in slack if it's taking a long time to release work you've done.