Does not update lockfile when monorepo packages are published

[freaktechnik]

The lockfile is not updated when multiple packages from a monorepo are updated in a single branch, since it expects there only to be one commit, however there are multiple commits.

Example:
freaktechnik/justintv-stream-notifications#425

[janl]

heya @freaktechnik, two options:

1. If you only have public dependencies, Greenkeeper now updates lockfiles automatically:https://blog.greenkeeper.io/announcing-native-lockfile-support-85381a37a0d0

2. if you have private deps, you still need greenkeeper-lockfile to opt into monorepo support, you need to install version 2 explicitly, here:

https://github.com/freaktechnik/justintv-stream-notifications/blob/master/.travis.yml#L25

before_install: npm i -g greenkeeper-lockfile@2

See #164 for details.