fix: 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