[Gemfile] downgrade ffi to 1.12.2 (require ruby >= 2.3)

Gemfile.lock

@@ -43,7 +43,7 @@ GEM
       http-cookie (~> 1.0.0)
     faraday_middleware (0.13.1)
       faraday (>= 0.7.4, < 1.0)
-    ffi (1.13.1)
+    ffi (1.12.2)
     hash_validator (1.0.0)
     hashdiff (1.0.0)
     http-cookie (1.0.3)