Version 3.1.1

Version 3.1.1 on RubyGems

Release notes:

• Add BCrypt::Password.valid_hash?(str) to check if a string is a valid bcrypt password hash
• Cost should be set to DEFAULT_COST if nil
• Add BCrypt::Engine.cost attribute for getting/setting a default cost externally
• Support removed for Ruby 1.8 in compiled win32 binaries