gistfile1.rb

#!/usr/bin/env ruby
revisions = [
  '1.1',
  '1.2',
  '1.3',
  '1.3.2.1',
  '1.3.2.3',
  '1.3.2.4',
  '1.3.4.1',
  '1.4',
  '1.5'
]

branch = "1.3.2"
puts revisions.select{|v| v =~ /#{branch}\.\d+$/}.max{|a,b| a.split('.').last.to_i <=> b.split('.').last.to_i}

# Output is: 1.3.2.4