fix node fetch vulnerability

[okhiroyuki]

Update isomorphic-fetch to eliminate a node-fetch vulnerability.

refs: #31

test

yarn test
yarn run v1.22.10
$ mocha


  English TTS
    ✓ Hello (101ms)
    ✓ hello
    ✓ hello world
    ✓ 123

  Long Characters
    ✓ English: 180 characters
    ✓ English: 200 characters
    ✓ English: 268 characters throw RangeError
    ✓ Chinese: 193 characters
    ✓ Chinese: 200 characters
    ✓ Chinese: 211 characters throw RangeError

  parameters
    ✓ text = null (566ms)
    ✓ text = '' (201ms)
    ✓ text = 123 (186ms)
    ✓ lang = null (191ms)
    ✓ lang = '' (198ms)
    ✓ lang = 123 (number) (193ms)
    ✓ speed = null (188ms)
    ✓ speed = '123' (226ms)
    ✓ timeout = 10 ms (too short to success)

  Chinese TTS
    ✓ 你好
    ✓ 你好世界
    ✓ 123


  22 passing (2s)

✨  Done in 4.75s.

[coveralls]

Coverage remained the same at 96.364% when pulling 7be0b40 on okhiroyuki:fix/node-fetch-vulnerability into b55689f on zlargon:master.

[zlargon]

@okhiroyuki, thanks for helping solve the vulnerability. The vulnerability issue has been fixed in the latest version 0.0.5.
Also, the test framework was changing from Mocha to Jest for better supporting with latest node version.
Thanks.