Recognize more language constructs: regexp, dstr, yield (fixes #173).

Of course, (regexp ...) (dstr ...) and (yield) are actually pretty common.
The virtual machine has ignored these constructs but it has worked
in most cases because child nodes provided the value.

```rb
puts //            # (regexp (regopt))
/foo/i             # (regexp (str "foo") (regopt :i))
puts <<EOS         # (dstr)
EOS
puts "foo#{bar}"   # (dstr (str "foo") (begin (send nil :bar)))
```

lib/ruby-lint/variable_predicates.rb

@@ -20,6 +20,7 @@ module VariablePredicates
     #
     RUBY_CLASSES = {
       :str    => 'String',
+      :dstr   => 'String',
       :sym    => 'Symbol',
       :int    => 'Fixnum',
       :float  => 'Float',

lib/ruby-lint/virtual_machine.rb

@@ -103,7 +103,9 @@ class VirtualMachine < Iterator
       :int,
       :float,
       :str,
+      :dstr,
       :sym,
+      :regexp,
       :true,
       :false,
       :nil,
@@ -448,6 +450,13 @@ def on_self
       push_value(method.return_value)
     end
 
+    ##
+    # Pushes the return value of the block yielded to, that is, an unknown one.
+    #
+    def on_yield
+      push_unknown_value
+    end
+
     ##
     # Creates the definition for a module.
     #