Add an option to keep non-go files to `dep`

[mandrigin]

Problem

Currently dep prune removes non-go files from all the packages. That doesn't work for us because some packages we use have C sources in it. In the next release of dep there would be an option to keep non-go files intact.

When it is released, let's use it and use dep prune to keep our vendor folder nice and slim.

Acceptance Criteria

1. Essential non-go files aren't touched when dep prune is called.
2. Gopkg.toml has [prune] section with options set.
3. Updated (after dep prune) vendor folder is committed. Tests are removed, unused packages are removed.
4. Tests pass and the library still builds.

[dshulyak]

was implemented in #551