Invalidate the root /index.html when invalidating

When using the cloudfront_invalidate_root config option, it will correctly
invalidate all default root objects from /index.html to /, however this causes
an issue on the root /index.html for the website, as some browsers redirect
http://example.org/ to http://example.org which will request the /index.html and
as this hasn't been invalidated will serve the cached index.html.

This change just adds /index.html to the paths to be invalidated if the config
option is set to true

This helps somewhat with the issue #106
Add spec for invalidating the root index html

See https://github.com/laurilehmijoki/s3_website/pull/130 for discussion
Shorten: Option.exists -> Option.contains
Release 2.6.1
Add Tom Bell to contributors