[Travis CI Build Failing] Update Node.js versions for the Travis CI Build

## Problem
Travis CI build is failing on Node.js `v0.10`, `v0.12` which are no longer supported versions since this commit https://github.com/indutny/bn.js/commit/b5c0c423d69f56af84b3c7f14cf839907aab7b8b 

Details: https://github.com/nodejs/Release#end-of-life-releases

## Solution
How about removing CI tests for Node.js `v0.10`, `v0.12`? @dcousens 