Update bcrypt to 3.1.15

Bcrypt 3.1.11 has a bug which can cause unhashed errro
see https://stackoverflow.com/questions/50213071/why-does-bcrypt-no-longer-accept-hashes

Gemfile.lock

@@ -91,7 +91,7 @@ GEM
       thread_safe (~> 0.3, >= 0.3.1)
     babosa (1.0.2)
     base32 (0.3.2)
-    bcrypt (3.1.11)
+    bcrypt (3.1.15)
     bcrypt_pbkdf (1.0.0)
     benchmark-ips (2.3.0)
     better_errors (2.1.1)