Fix safe Style/ColonMethodCall offenses

Caxlsx is using both `.` and `::`, 220 occurrences vs 280 to invoke
methods on `Axlsx` module.

This commit standardizes the approach towards `.`, which will also allow
shorter lines.

Performance is not affected

```
Comparison:
      Axlsx.validate:  8515252.3 i/s
     Axlsx::validate:  8512863.7 i/s - same-ish: difference falls within error 
```

1 files changed, 5 insertions, 5 deletions

diff --git a/lib/axlsx/drawing/axis.rb b/lib/axlsx/drawing/axis.rb
index dfef7890..ed3df5f1 100644
--- a/lib/axlsx/drawing/axis.rb
+++ b/lib/axlsx/drawing/axis.rb
@@ -109,15 +109,15 @@ module Axlsx
 
     # The number format format code for this axis
     # default :General
-    def format_code=(v) Axlsx::validate_string(v); @format_code = v; end
+    def format_code=(v) Axlsx.validate_string(v); @format_code = v; end
 
     # Specify if gridlines should be shown for this axis
     # default true
-    def gridlines=(v) Axlsx::validate_boolean(v); @gridlines = v; end
+    def gridlines=(v) Axlsx.validate_boolean(v); @gridlines = v; end
 
     # Specify if axis should be removed from the chart
     # default false
-    def delete=(v) Axlsx::validate_boolean(v); @delete = v; end
+    def delete=(v) Axlsx.validate_boolean(v); @delete = v; end
 
     # specifies how the perpendicular axis is crossed
     # must be one of [:autoZero, :min, :max]
@@ -126,9 +126,9 @@ module Axlsx
     # Specify the degree of label rotation to apply to labels
     # default true
     def label_rotation=(v)
-      Axlsx::validate_int(v)
+      Axlsx.validate_int(v)
       adjusted = v.to_i * 60000
-      Axlsx::validate_angle(adjusted)
+      Axlsx.validate_angle(adjusted)
       @label_rotation = adjusted
     end