Speed up rehash
Speed up rehash

Before (20 ruby versions, producing 200 shims total):

    mean:    0.124 s
    stdev:   0.003

After:

    mean:    0.090 s
    stdev:   0.004

sstephenson/rbenv,sstephenson/rbenv,rbenv/rbenv,rbenv/rbenv,sstephenson/rbenv