Marked is a dependency of jsdoc. Added marked as a dev dependency to
be able to update it to latest version. This is just a temporary fix
until jsdoc/jsdoc#1489 has been released.

Idea from jsdoc/jsdoc#1489 (comment)

Related to #22

https://nvd.nist.gov/vuln/detail/CVE-2017-1000427

I'm not sure this works or if you have to specify an exact version
for "allow_failures". Anyway, this is just a temporary fix until #24 has
been solved.

Also cleaned up the version we test against:
* 5.x and 7.x removed as their EOL was at 2016-06-30
* 8.x is the current active LTS release

(see https://github.com/nodejs/Release)