Update ruby-bcrypt to 3.1.2.

3.1.0  May 07 2013
  - Add BCrypt::Password.valid_hash?(str) to check if a string is a valid
    bcrypt password hash
  - BCrypt::Password cost should be set to DEFAULT_COST if nil
  - Add BCrypt::Engine.cost attribute for getting/setting a default cost
    externally

3.1.1  Jul 10 2013
  - Remove support for Ruby 1.8 in compiled win32 binaries

3.1.2  Aug 26 2013
  - Add support for Ruby 1.8 and 2.0 (in addition to 1.9) in compiled Windows
    binaries
  - Add support for 64-bit Windows

(taca)