Fix install issue due to drollup

[phaberest]

The @next version of https://github.com/cmorten/deno-rollup does not provide any package when creating a new Snel project, resulting in the error

error: Module not found "https://denopkg.com/buttercubz/deno-rollup@next/plugins/terser/mod.ts". at https://deno.land/x/[email protected]/src/shared/internal_plugins.ts:13:24

This should hopefully fix the issue

Fixes #59