fix(travis): node js version should between quote

node js version should between quotation marks based on http://docs.travis-ci.com/user/languages/javascript-with-nodejs/

this my error build before https://travis-ci.org/saiqulhaq/kalkulator-ipk/builds/31290985
and this is success build after i put quotation https://travis-ci.org/saiqulhaq/kalkulator-ipk/builds/31291571