Merge???
Fix SUPPRESSINSTALL typo

Summary:
Luke added an option to skip running `npm install` for any
Makefile command, which might be useful. This commit fixes
a typo in it.

Test Plan:
`make server`
see that it doesn't warn about the following anymore:

    Makefile:78: warning: undefined variable `SUPPRESSINSTALL'

Auditors: michelle
Merge branch 'master' of https://github.com/Khan/perseus