TestSpec is not really flaky

[KyleMaas]

See #237

You can't run the tests without cloning envelope-spec into the test directory:

=== RUN   TestSpec
    spec_test.go:29:
                Error Trace:    /home/go-ssb/go-ssb/private/box2/spec_test.go:29
                Error:          Received unexpected error:
                                open spec/vectors: no such file or directory
                Test:           TestSpec
                Messages:       open vectors dir
    spec_test.go:30: suggestion: run 'git clone https://github.com/ssbc/envelope-spec spec'
    spec_test.go:31: or clone it somewhere else and create a symlink to it here named 'spec'
--- FAIL: TestSpec (0.00s)
FAIL
FAIL    github.com/ssbc/go-ssb/private/box2     0.008s
FAIL

Not sure if there's a way to integrate this into the test suite or if the CI system downloads this when testing. Maybe this is why it was failing before? I ran it about 150 times and had zero failures once I downloaded envelope-spec.