Don't run on Node 0.12 in Travis

0.12 reached end-of-life at the end of 2016 (see:
https://github.com/nodejs/LTS).

It is now failing CI runs too, presumably because of a Yarn upgrade on
the CI boxes, which now complain:

> Node.js version v0.12.18 does not meet requirement for yarn. Please
> use Node.js 4 or later.

(Example: https://travis-ci.org/graphql/graphql-js/jobs/214455818)

So, let's drop it from the CI matrix.

Author: wincent

.travis.yml

@@ -5,7 +5,6 @@ node_js:
   - '7'
   - '6'
   - '4'
-  - '0.12'
 
 git:
   depth: 5