“ We do not believe that the parole condition restricting association was intended to apply to incidental contacts between ex-convicts in the course of work on a legitimate job for a common employer. Nor is such occupational association, standing alone, satisfactory evidence of nonbusiness association violative of the parole restriction. To so assume would be to render a parolee vulnerable to imprisonment whenever his employer, willing to hire ex-convicts, hires more than one. ”