fix issue #79: add /v6 module import suffix

[mwf]

@sohamkamani I've added /v6 import suffix to resolve issue #79 introduced with v5.1.0 release with #40.

According to https://github.com/golang/go/wiki/Modules#why-must-major-version-numbers-appear-in-import-paths the major version must present in import paths.

If you wish, I could do it in v5 module, to fix the buggy v5.1.0 version, but it's strongly recommended to increase major version after adopting go modules: https://github.com/golang/go/wiki/Modules#incrementing-the-major-version-when-first-adopting-modules-with-v2-packages, so I've made it v6.

And please have a look at https://github.com/golang/go/wiki/Modules#releasing-modules-v2-or-higher
In short, there could be some problems with godep users with major go.mod in master. So we could introduce better compatibility with v6 subdirectory for non-gomod users.
But if you dropped Gopkg.toml support, maybe it's unnecessary.

Let's discuss an fix it ASAP to have better go.mod support 🙇

[coveralls]

Coverage remained the same at 100.0% when pulling 4ba367d on mwf:fix-gomod-module into 1aedfb0 on gojek:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 4ba367d on mwf:fix-gomod-module into 1aedfb0 on gojek:master.

[rShetty]

@mwf Thanks for the PR. This makes sense. Merging this.

[rShetty]

@mwf Can you also help change the README to import the package with the new changes?