Singularize time unit in client banner build age

This is a simple cosmetic patch.

In non-release builds of Julia, the client banner shows a commit string with the age of the build.

    Commit dc5350b* (1 days old master)

When one day old, the plural form of the time unit is still used. This patch simply chooses the correct form of "day" depending on the count.

    Commit dc5350b* (1 day old master)