Tag command produces error with only 1 version

[Leandro Silva]

• I was not able to find an open or closed issue matching what I'm seeing.
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Provide a narrative description of what you are trying to accomplish.

If the CHANGELOG has only 1 version, the tag command fails. Same behaviour with any version number (eg. 3.0.0, etc). Will investigate this issue and create a PR.

Code to reproduce the issue

$ vendor/bin/keep-a-changelog new
Created new changelog in file "/private/tmp/changelog-test/CHANGELOG.md" using initial version "0.1.0".
$ vendor/bin/keep-a-changelog entry:added "Testing"
Preparing entry for Added section
Writing "Added" entry to /private/tmp/changelog-test/CHANGELOG.md
$ git add CHANGELOG.md .gitignore composer.json
...
$ git commit -m "Test 1"
...
$ git push
...
$ vendor/bin/keep-a-changelog ready
Setting release date of most recent changelog to "2018-04-19"
$ vendor/bin/keep-a-changelog tag 0.1.0
In InvalidChangelogFormatException.php line 18:
  Changelog entry found for version 0.1.0, but it appears to be formatted incorrectly.
$ vendor/bin/keep-a-changelog bump
Bumped changelog version to 0.1.1
$ vendor/bin/keep-a-changelog ready
Setting release date of most recent changelog to "2018-04-19"
$ vendor/bin/keep-a-changelog tag 0.1.1
Created tag "0.1.1" for package "changelog-test" using the following notes:
Added
-----
...

Expected results

Tag working with the first version.

Actual results

Error produced above.