Travis updates

[harrysarson]

Updates Travis:

• Reverts changes in Help greenkeeper to manage lockfiles #1189 as greenkeeper is now smart enough not to need them.
• Uses named jobs in Travis rather than dummy environment variables.

References:

• https://blog.greenkeeper.io/announcing-native-lockfile-support-85381a37a0d0
• https://docs.travis-ci.com/user/customizing-the-build/#naming-jobs-within-matrices

[harrysarson]

I confirmed in #1246 that green-keeper still works with these changes 👍

[josdejong]

wow that's a much simpler config 👍