Refactor: override implies virtual

If not used with `final`, override implies `virtual`.
Detected via CodeFactor score.

Another reference on SO:
  https://stackoverflow.com/questions/43466863/isnt-virtual-keyword-redundant-when-override-or-final-specifiers-are-used

docs/matchers.md

@@ -92,7 +92,7 @@ public:
     IntRange( int begin, int end ) : m_begin( begin ), m_end( end ) {}
 
     // Performs the test for this matcher
-    virtual bool match( int const& i ) const override {
+    bool match( int const& i ) const override {
         return i >= m_begin && i <= m_end;
     }