chore: Pass Node versions as strings to Travis

Node versions, as most versions, are strings, not numbers. 0.10 as a number
equals 0.1 which by the virtue of nvm is matched as 0.10 but shouldn't be
relied upon. Also, it screws Node numbering in Travis runs, see e.g.:
https://travis-ci.org/karma-runner/grunt-karma/builds/28585432
Merge pull request #118 from mzgol/travis-node-version

chore: Pass Node versions as strings to Travis
fix: Pass node-specific command line params to the subprocess

This commit makes the karma task pass Node-specific parameters used to start
the Node process to its background subprocess. This is required e.g. to be able
to use the --harmony flag.

Similar pull request for Grunt itself (already merged):
https://github.com/gruntjs/grunt/pull/877