Avoid choking on Ruby 2.3 heredoc syntax

Ruby 2.3 introduces `<<~` heredoc syntax, but when ripper-tags is ran on
Ruby 2.2, it acts weirdly around the heredoc, passing a `:~` symbol to
`on_stmts_add`.

This change avoids Ruby chrashing when encountering this syntax which it
doesn't understand.

lib/ripper-tags/parser.rb

@@ -17,6 +17,7 @@ def on_#{event}(tok)
   end
 
   def on_stmts_add(first, *rest)
+    return if first == :~
     (first || []) + rest.compact
   end
 

test/test_ripper_tags.rb

@@ -434,4 +434,20 @@ class A
     assert_equal 1, tags.size
     assert_equal 'A', tags[0][:name]
   end
+
+  def test_heredoc
+    tags = extract(<<-EOC)
+      def foo
+        puts "hello", <<~EOF
+          world
+        EOF
+      end
+
+      def bar; end
+    EOC
+
+    assert_equal 2, tags.size
+    assert_equal 'foo', tags[0][:name]
+    assert_equal 'bar', tags[1][:name]
+  end
 end