Remove old versions of Node.js from .travis.yml + Update Deps

We have a couple of ancient versions of node.js in the .travis.yml file:

aws-sdk-mock/.travis.yml

Lines 2 to 7 in f78025f

Also, we have "node" which is just the "current" version, i.e. not very explicit.
It caused a break in a recent PR build: #236 (comment)

Going to remove the older versions and the non-specific one.

At the time of writing v12 and v14 are the supported LTS versions of Node: https://nodejs.org/en/about/releases

So I'm removing the others as they are no longer supported. ✂️

	node_js:
	- "node"
	- "8"
	- "10"
	- "12"
	- "14"