Travis config: Drop testing node v0.8.x
Travis config: Drop testing node v0.8.x

[Apparently][1] that previous commit's build only failed on Node.js
version 0.8.

I for one can't be bothered to figure out what bug in npm made it fail
(I assume it's that), so I suggest we drop testing for 0.8 altogether.
The last maintenance to v0.8.x is [mid-2014][2] and [the upgrade to 0.12
broke very little API anyway][3].

[1]: https://travis-ci.org/jneen/parsimmon/builds/99770512
[2]: https://nodejs.org/en/blog/release/v0.8.28/
[3]: https://strongloop.com/strongblog/node-js-v0-12-apis-breaking/

jneen/parsimmon,anko/partser