update filecoin-ffi in go mod

[jsign]

Currently, the go.mod has an old version of this dependency.
For this repo isn't a problem since it has a replace directive, but for any client go mod tidy fails.

Minimal way to reproduce the problem now:

package main

import (
        "context"
        "github.com/filecoin-project/lotus/api/apistruct"
)

func main() {
        var api apistruct.FullNodeStruct
        _ = api
  }

Run go mod tidy:

go: finding module for package github.com/filecoin-project/specs-actors/actors/abi
github.com/jsign/lotusclientest imports
        github.com/filecoin-project/lotus/api/apistruct imports
        github.com/filecoin-project/go-fil-markets/storagemarket tested by
        github.com/filecoin-project/go-fil-markets/storagemarket.test imports
        github.com/filecoin-project/go-fil-markets/pieceio imports
        github.com/filecoin-project/filecoin-ffi imports
        github.com/filecoin-project/specs-actors/actors/abi: module github.com/filecoin-project/specs-actors@latest found (v0.9.13), but does not contain package github.com/filecoin-project/specs-actors/actors/abi

Generated go.mod:

module github.com/jsign/lotusclientest

go 1.15

require (
        github.com/filecoin-project/lotus v1.1.2
)

In Poweragte I work around this with a replace, but other users might not have a clue of how to fix it. Also, if anyone else is using Powergate as a lib they should also replace in their go.mod since replaces doesn't bubble up modules, which is a bigger problem.

This PR fixes this problem.