Rename 'RGBColor' to 'RgbColor'

Due to the way the package name is handled in the metadata, `RGBColor`
causes some attributes to become `rgbcolor` rather than `rgb_color` (for
the latter `RgbColor` is required).

lib/DDG/Goodie/RgbColor.pm

@@ -1,4 +1,4 @@
-package DDG::Goodie::RGBColor;
+package DDG::Goodie::RgbColor;
 
 use DDG::Goodie;
 use strict;

t/RgbColor.t

@@ -29,7 +29,7 @@ sub build_structured_answer {
 sub build_test { test_zci(build_structured_answer(@_)) }
 
 ddg_goodie_test(
-    [qw( DDG::Goodie::RGBColor )],
+    [qw( DDG::Goodie::RgbColor )],
     'example query' => build_test('query'),
     'bad example query' => undef,
 );