Don't gitignore changelog so it's kept with the distribution (#64)

Author: mfn

.gitattributes

@@ -4,7 +4,6 @@
 /.gitattributes export-ignore
 /.github export-ignore
 /.travis.yml export-ignore
-/CHANGELOG.md export-ignore
 /Makefile export-ignore
 /phpunit.xml.dist export-ignore
 /tests export-ignore