Change password_hash() algorithm from CRYPT_BLOWFISH to PASSWORD_BCRYPT.

Summary:
PHP 5.5 specifies constant PASSWORD_BCRYPT should be used in password_hash()
instead of CRYPT_BLOWFISH. Using CRYPT_BLOWFISH is not supported in either PHP
or HHVM. This constant breaks Username / Password authentication.

Test Plan:
Login using Username/Password with bcrypt hash. Before applying the patch,
No matter what password entered, it will always fail authentication. After this
patch, user should be able to login with bcrypt hash.

Reviewers: btrahan, epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D8808