Remove pegjs from package.json
Remove pegjs from package.json

It looks like the event parser generator was moved into separate dependency in https://github.com/vega/vega/commit/9792c3d947c41c4434c68c4daf57ef5719f7db2b , but the pegjs build process is still part of the build process, which results in a build error:

```
16:24 vega$ npm run build

> vega@2.4.2 build /opt/src/vega/vega
> npm run pegjs && browserify index.js -d -s vg | exorcist vega.js.map > vega.js


> vega@2.4.2 pegjs /opt/src/vega/vega
> pegjs src/parse/events.pegjs

(node) util.error is deprecated. Use console.error instead.
Can't read from file "src/parse/events.pegjs".

```

Perhaps Travis should be configured to try to run the full build to catch this sort of problem in the future?
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