Make git-changelog explicitly use python2 in shebang
Make git-changelog explicitly use python2 in shebang

git-changelog was breaking on systems where python = python3, so update
the shebang to use python2 explicitly

Closes #143

unixorn/git-extra-commands,unixorn/git-extra-commands,unixorn/git-extra-commands,unixorn/git-extra-commands