[INFRA] downgrad github-changelog-generator to 1.14.3

Due to --since-tag issue. See:
bids-standard#594 (comment)
And investigation here:
https://gitter.im/github-changelog-generator/chat?at=5f57456cec534f584fedf00d
This param gets automatically set when using
--base, because the "base" file gets parsed for versions. In our case,
the latest version in "base" (pregh-changes.md) happens to be 1.1.1,
and then --since-tag is automatically set to 1.1.1 and that tag
cannot be found in the repository, because we never tagged such a
version in git.

Alternative options next to downgrading github-changelog-generator
are (i) to make a PR against github-changelog-generator and add
an option that allows explicitly setting --since-tag to None, or
(ii) tagging the first commit of the bids-specification repository
as 1.1.1, however the latter would be a weird hack.

.circleci/config.yml

@@ -66,7 +66,7 @@ jobs:
   github-changelog-generator:
     working_directory: ~/build
     docker:
-      - image: ferrarimarco/github-changelog-generator:1.15.2
+      - image: ferrarimarco/github-changelog-generator:1.14.3
     steps:
       - setup_remote_docker:
            version: 18.06.0-ce