fix: Suppress Tilt warning

Generating stats with Haml6 raises the following warning:
```
Haml::TempleEngine: Option :line is invalid
```

By loading haml before tilt, the warning is suppressed
because it uses `Haml::Template` provided by Haml6.

lib/git_stats.rb

@@ -8,6 +8,7 @@ def self.root
 
 require 'active_support/all'
 require 'fileutils'
+require 'haml'
 require 'tilt'
 require 'pathname'
 require 'lazy_high_charts'