[README] update build instructions
[README] update build instructions

- update documented PureScript version
- add missing `npm install` command
- change `gulp build` to `gulp make`

Relates to issue #144 and PR #330 (see
https://github.com/slamdata/slamdata/pull/330#discussion_r36101093).

cryogenian/slamdata,jonsterling/slamdata,shujuwen2013/slamdata,wemrysi/slamdata,drostron/slamdata,mossprescott/slamdata,beckyconning/slamdata,natefaubion/slamdata,mossprescott/slamdata,zeeiyerWork/slamdata,cryogenian/slamdata,beckyconning/slamdata,natefaubion/slamdata,sellout/slamdata,natefaubion/slamdata,drostron/slamdata,jonsterling/slamdata,shujuwen2013/slamdata,zeeiyerWork/slamdata,wemrysi/slamdata,sellout/slamdata