add pvp versionScheme to project POMs

[bpholt]

PVP is described by sbt as a version scheme in which "X.Y are treated as [the] major version," which I think accurately describes the version scheme of this library. Adding this setting to a library's POM will help sbt manage evictions and transitive dependencies, and should help avoid binary incompatibilities. (See "Preventing Version Conflicts with versionScheme" and the sbt documentation for more details.)

If this change is accepted, I'm happy to make similar PRs to finagle and scrooge.