fix package.json to not use caret operator

older versions of npm, of which travis uses, are not compatible with the
new caret style for npm semvars

so i’m changing this one back to a tilde so that we don’t break our 
customers, or travis builds

fyi this could also be done to fix travis:

http://tech.vg.no/2014/05/30/npm-travis-node-and-the-caret-operator/