Rename Severity::Refactoring -> Severity::Convention

it preserves clang format

spec/ameba/formatter/dot_formatter_spec.cr

@@ -74,7 +74,7 @@ module Ameba::Formatter
           end
           subject.finished [s]
           log = output.to_s
-          log.should contain "[R]"
+          log.should contain "[C]"
         end
 
         it "doesn't write affected code if it is disabled" do

spec/ameba/formatter/explain_formatter_spec.cr

@@ -50,7 +50,7 @@ module Ameba
         source = Source.new "a = 42", "source.cr"
         output = explanation(source)
         output.should contain "RULE INFO"
-        output.should contain "Refactoring"
+        output.should contain "Convention"
         output.should contain "Ameba/ErrorRule"
         output.should contain "Always adds an error at 1:1"
       end

spec/ameba/formatter/flycheck_formatter_spec.cr

@@ -22,7 +22,7 @@ module Ameba::Formatter
         subject = flycheck
         subject.source_finished s
         subject.output.to_s.should eq(
-          "source.cr:1:2: R: [#{DummyRule.rule_name}] message\n"
+          "source.cr:1:2: C: [#{DummyRule.rule_name}] message\n"
         )
       end
 
@@ -32,7 +32,7 @@ module Ameba::Formatter
         subject = flycheck
         subject.source_finished s
         subject.output.to_s.should eq(
-          "source.cr:1:2: R: [#{DummyRule.rule_name}] multi line\n"
+          "source.cr:1:2: C: [#{DummyRule.rule_name}] multi line\n"
         )
       end
 

spec/ameba/formatter/json_formatter_spec.cr

@@ -42,7 +42,7 @@ module Ameba
         s.add_issue DummyRule.new, {1, 2}, "message"
 
         result = get_result [s]
-        result["sources"][0]["issues"][0]["severity"].should eq "Refactoring"
+        result["sources"][0]["issues"][0]["severity"].should eq "Convention"
       end
 
       it "shows a message" do

spec/ameba/formatter/todo_formatter_spec.cr

@@ -44,7 +44,7 @@ module Ameba
       end
 
       it "creates a todo with severity" do
-        create_todo.should contain "Refactoring"
+        create_todo.should contain "Convention"
       end
 
       it "creates a todo with problems count" do