Fixing prolog lexer issue on camelCase atoms

Although camelCase atoms - as well as functor's and predicate's names -
are sometimes considered a bad practice, they are actually valid.

Before this commit, a camelCaseAtom would be highlighted as **camel**CaseAtom.
Now, it is properly highlighted as **camelCaseAtom**

lib/rouge/lexers/prolog.rb

@@ -30,7 +30,7 @@ module Rouge
       end
       state :atoms do
-        rule /[[:lower:]]([_[:lower:][:digit:]])*/, Str::Symbol
+        rule /[[:lower:]]([_[:word:][:digit:]])*/, Str::Symbol
         rule /'[^']*'/, Str::Symbol
       end

spec/visual/samples/prolog

@@ -4,6 +4,7 @@ x
 blue
 'Burrito'
 'some atom'
+camelCaseAtom
 # Numbers