Fix keyword argument warnings for specs for Ruby 2.7 (#15)

When running specs via `RUBYOPT="-W:deprecated" rake` Ruby 2.7 emits
with following warnings:

<path>/test/simplecov-cobertura_test.rb:12:
warning: Passing the keyword argument as the last hash parameter is
deprecated
<path>/installs/ruby/2.7.2/lib/ruby/gems/2.7.0/gems/simplecov-0.19.1/lib/simplecov/result.rb:28:
warning: The called method `initialize' is defined here

This commit fixes the warning.

See also
https://www.ruby-lang.org/en/news/2019/12/12/separation-of-positional-and-keyword-arguments-in-ruby-3-0/

test/simplecov-cobertura_test.rb

@@ -7,9 +7,8 @@
 require 'simplecov-cobertura'
 
 class CoberturaFormatterTest < Test::Unit::TestCase
-
   def setup
-    @result = SimpleCov::Result.new("#{__FILE__}" => [1,2])
+    @result = SimpleCov::Result.new({ "#{__FILE__}" => [1,2] })
     @formatter = SimpleCov::Formatter::CoberturaFormatter.new
   end