Revert nvmrc to a widely compatible string (#249)

Hello from Railsconf! 👋 This closes #248 by updating the specifier in
`.nvmrc` to one compatible with nodenv and asdf, neither of which
(unfortunately) support the `lts/*` specifier. (Neither of these tools
will read `.nvmrc` anyway, I'm assuming an entry point of `bin/setup`.)

Author: jas14

.nvmrc

@@ -1 +1 @@
-lts/*
+20.12.2