add 'make update' to makefile

[osterbit]

I use this very simple shell script to update my qri repos, may not be quite generic enough to copy paste, but thought I'd share as a starting point to make it easier to update everything

qri_update.sh:

cd ${GOPATH}/src/github.com/qri-io/qri
git checkout master
git pull
gx install
cd ../cafs
git checkout master
git pull
cd ../dataset
git checkout master
git pull
cd ../varName
git checkout master
git pull
cd ../datasetDiffer
git checkout master
git pull
cd ../dataset_sql
git checkout master
git pull
cd /Users/osterbit/sdev/web
git checkout master
git pull
yarn
cd ${GOPATH}/src/github.com/qri-io/qri