Version bump postgres-nio dependency to prevent build error against swift-nio

This patch was authored by @eob and released by @gwynne.

Hi folks!

I'm not sure what this project's process for version-bumping is, but the current vapor/[email protected] dependency can't build against the more current swift-nio versions because of a duplicate addition of writeNullTerminatedString to ByteBuffer by extension:

/path/to/.build/checkouts/postgres-nio/Sources/PostgresNIO/New/Extensions/ByteBuffer+PSQL.swift:4:19: note: found this candidate
    mutating func writeNullTerminatedString(_ string: String) {
                  ^
/path/to/.build/checkouts/swift-nio/Sources/NIOCore/ByteBuffer-aux.swift:100:26: note: found this candidate
    public mutating func writeNullTerminatedString(_ string: String) -> Int { 

The postgres-nio project has since removed the duplicate method definition, so rolling forward the dependency to 1.7.2 (in this PR) fixes the build error.