[ruby/rack] Mark as platform (TechEmpower#9723)

All Ruby frameworks are build on Rack, so the classification should be
considered Platform instead of Micro.
It's even mentioned in the wiki:

> ... a web platform such as Rack, Servlet, or plain PHP

https://github.com/TechEmpower/FrameworkBenchmarks/wiki/Project-Information-Basic-Concepts

Author: p8

frameworks/Ruby/rack/benchmark_config.json

@@ -11,7 +11,7 @@
         "update_url": "/updates?queries=",
         "port": 8080,
         "approach": "Realistic",
-        "classification": "Micro",
+        "classification": "Platform",
         "orm": "raw",
         "database": "Postgres",
         "framework": "rack",
@@ -32,7 +32,7 @@
         "update_url": "/updates?queries=",
         "port": 8080,
         "approach": "Realistic",
-        "classification": "Micro",
+        "classification": "Platform",
         "orm": "raw",
         "database": "Postgres",
         "framework": "rack",
@@ -53,7 +53,7 @@
         "update_url": "/updates?queries=",
         "port": 8080,
         "approach": "Realistic",
-        "classification": "Micro",
+        "classification": "Platform",
         "orm": "raw",
         "database": "Postgres",
         "framework": "rack",
@@ -74,7 +74,7 @@
         "update_url": "/updates?queries=",
         "port": 8080,
         "approach": "Realistic",
-        "classification": "Micro",
+        "classification": "Platform",
         "orm": "raw",
         "database": "Postgres",
         "framework": "rack",
@@ -95,7 +95,7 @@
         "update_url": "/updates?queries=",
         "port": 8080,
         "approach": "Realistic",
-        "classification": "Micro",
+        "classification": "Platform",
         "orm": "raw",
         "database": "Postgres",
         "framework": "rack",