Fix StringConcatenation offenses (non-production)

Prefer interpolation over concatenation

```
Comparison ("String#{'String'}" vs 'String' + 'String'):
       interpolation: 11821321.0 i/s
       concatenation:  8849491.7 i/s - 1.34x  (± 0.00) slower
```

1 files changed, 2 insertions, 2 deletions

diff --git a/Rakefile b/Rakefile
index 8125959f..28712c0d 100644
--- a/Rakefile
+++ b/Rakefile
@@ -1,13 +1,13 @@
 # frozen_string_literal: true
 
-require File.expand_path(File.dirname(__FILE__) + '/lib/axlsx/version.rb')
+require File.expand_path("#{File.dirname(__FILE__)}/lib/axlsx/version.rb")
 
 task build: :gendoc do
   system "gem build axlsx.gemspec"
 end
 
 task :benchmark do
-  require File.expand_path(File.dirname(__FILE__) + '/test/benchmark.rb')
+  require File.expand_path("#{File.dirname(__FILE__)}/test/benchmark.rb")
 end
 
 task :gendoc do