fix writing proofs to output files

Fixes e.g. `keybase prove github -o github_proof.txt`, which was giving me a traceback like the following:

```TypeError: Object #<GithubProofGen> has no method 'get_proof_text'
    at prompt (/usr/local/lib/node_modules/keybase/lib/command/prove.js:342:41)
    at prompt (/usr/local/lib/node_modules/keybase/lib/command/prove.js:346:15)
    at Command.exports.Command.Command.handle_post (/usr/local/lib/node_modules/keybase/lib/command/prove.js:353:15)
    at /usr/local/lib/node_modules/keybase/lib/command/prove.js:589:39
    at /usr/local/lib/node_modules/keybase/lib/command/prove.js:593:33
    at Deferrals.exports.Deferrals.Deferrals._call (/usr/local/lib/node_modules/keybase/node_modules/iced-runtime/lib/runtime.js:86:16)
    at /usr/local/lib/node_modules/keybase/node_modules/iced-runtime/lib/runtime.js:98:26
    at exports.trampoline.trampoline (/usr/local/lib/node_modules/keybase/node_modules/iced-runtime/lib/runtime.js:64:14)
    at Deferrals.exports.Deferrals.Deferrals._fulfill (/usr/local/lib/node_modules/keybase/node_modules/iced-runtime/lib/runtime.js:96:16)
    at ret (/usr/local/lib/node_modules/keybase/node_modules/iced-runtime/lib/runtime.js:29:18)```