fix: avoid emitting warnings when ruby is run with -w (#1)

I've also turned on warnings for this gem's tests, to make it more
obvious when this is happening in the future.

Without this fix, users will see repeated messages like this:

> source_file_formatter.rb:20: warning: instance variable @line_coverage not initialized

.rspec

@@ -0,0 +1 @@
+--warnings

lib/simplecov_json_formatter/source_file_formatter.rb

@@ -4,6 +4,7 @@ module SimpleCovJSONFormatter
   class SourceFileFormatter
     def initialize(source_file)
       @source_file = source_file
+      @line_coverage = nil
     end
 
     def format
@@ -17,7 +18,7 @@ def format
     private
 
     def line_coverage
-      @line_coverage || {
+      @line_coverage ||= {
         lines: lines
       }
     end