Update bigdecimal version used in benchmark from 3.0.0 to 3.1.1 (#416)

GitHub actions macos-latest has been updated to macos-15, and `gem install bigdecimal -v 3.0.0` began to fail.

Author: tompng

.github/workflows/benchmark.yml

@@ -53,7 +53,7 @@ jobs:
     - name: Install dependencies
       run: |
         bundle install
-        gem install bigdecimal -v 3.0.0
+        gem install bigdecimal -v 3.1.1
 
     - run: rake compile
 

benchmark/from_float.yml

@@ -2,7 +2,7 @@ loop_count: 100000
 
 contexts:
 - gems:
-    bigdecimal: 3.0.0
+    bigdecimal: 3.1.1
 - name: "master"
   prelude: |-
     $LOAD_PATH.unshift(File.expand_path("lib"))

benchmark/from_large_integer.yml

@@ -2,7 +2,7 @@ loop_count: 1000
 
 contexts:
 - gems:
-    bigdecimal: 3.0.0
+    bigdecimal: 3.1.1
 - name: "master"
   prelude: |-
     $LOAD_PATH.unshift(File.expand_path("lib"))

benchmark/from_small_integer.yml

@@ -2,7 +2,7 @@ loop_count: 100000
 
 contexts:
 - gems:
-    bigdecimal: 3.0.0
+    bigdecimal: 3.1.1
 - name: "master"
   prelude: |-
     $LOAD_PATH.unshift(File.expand_path("lib"))