chore(travis): modify travis.yml to support OS X
chore(travis): modify travis.yml to support OS X

Change language to c++ to delay nvm commands until after nvm is installed.
Install steps are explicit to make sure that Travis CI will always have the nvm
package, even on OS X. Changing to Node v4.x.x

psema4/shelljs,arturadib/shelljs,shelljs/shelljs,jcoreio/shelljs,freitagbr/shelljs