update deprecated requires

Was getting deprecation warnings:
```
Warning: Use of deprecated require path:
  In /Users/Nicholas/mozilla/fxos-version-control/packages/jetpack-subprocess/lib/subprocess.js:160:
    require('api-utils/environment').
  New path should be:
    require('sdk/system/environment')
Warning: Use of deprecated require path:
  In /Users/Nicholas/mozilla/fxos-version-control/packages/jetpack-subprocess/lib/subprocess.js:159:
    require('api-utils/runtime').
  New path should be:
    require('sdk/system/runtime')
```