Upgrade Probot version to support GHE

[issc29]

Currently, Prevent Public Repos Probot App does uses a version of Probot that is not compatible with GitHub Enterprise.

Recommend: Upgrade version of Probot from 3.0.0 to at least 4.0.0

From #4

[kreig303]

I am also interested in GHE support. I've identified the following before coding (per https://github.com/probot/probot/releases)

1. increment probot dependency to ^6.0.0
2. remove local-tunnel dev dependency
3. add smee-client dev dependency

Uncertain about other changes yet.

[kreig303]

I had a branch on my fork and am now PRing it.

Only additional change was linting fix.

#10

[kreig303]

Made some more tweaks... had to fix .travis.yml as it was using an inappropriate version of node and causing build failures.

[issc29]

Fixed in #10