* vendor: update Lumberjack dep
* httpserver/roller: introduce rotate_compress directive
This directive will enable gzip compression provided by [Lumberjack](https://github.com/natefinch/lumberjack/pull/43).
The directive `rotate_compress` can be `true` or `false`, being `false` by default.
* httpserver/roller: remove need to set bool with rotate_compress option
The vendor/ folder was created with the help of @FiloSottile's gvt and
vendorcheck.
Any dependencies of Caddy plugins outside this repo are not vendored.
We do not remove any unused, vendored packages because vendorcheck -u
only checks using the current build configuration; i.e. packages that
may be imported by files toggled by build tags of other systems.
CI tests have been updated to ignore the vendor/ folder. When Go 1.9 is
released, a few of the go commands should be revised to again use ./...
as it will ignore the vendor folder by default.
