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, 4 insertions, 4 deletions

diff --git a/lib/axlsx/drawing/pic.rb b/lib/axlsx/drawing/pic.rb
index 08beb2d9..630d8d96 100644
--- a/lib/axlsx/drawing/pic.rb
+++ b/lib/axlsx/drawing/pic.rb
@@ -78,7 +78,7 @@ module Axlsx
     end
 
     def image_src=(v)
-      Axlsx::validate_string(v)
+      Axlsx.validate_string(v)
       if remote?
         RegexValidator.validate('Pic.image_src', /\A#{URI::DEFAULT_PARSER.make_regexp}\z/, v)
         RestrictionValidator.validate 'Pic.image_src', ALLOWED_MIME_TYPES, MimeTypeUtils.get_mime_type_from_uri(v)
@@ -91,13 +91,13 @@ module Axlsx
     end
 
     # @see name
-    def name=(v) Axlsx::validate_string(v); @name = v; end
+    def name=(v) Axlsx.validate_string(v); @name = v; end
 
     # @see descr
-    def descr=(v) Axlsx::validate_string(v); @descr = v; end
+    def descr=(v) Axlsx.validate_string(v); @descr = v; end
 
     # @see remote
-    def remote=(v) Axlsx::validate_boolean(v); @remote = v; end
+    def remote=(v) Axlsx.validate_boolean(v); @remote = v; end
 
     def remote?
       remote == 1 || remote.to_s == 'true'