Rename Void to Void_ since it is a reserved keyword in PHP7.1

Fixing the followint failure:

1) phpDocumentor\Reflection\TypeResolverTest::testResolvingKeywords with
data set #15 ('void', 'phpDocumentor\Reflection\Types\Void')
PHPUnit_Framework_Exception: Argument #1 (No Value) of
PHPUnit_Framework_Assert::assertInstanceOf() must be a class or
interface name

.travis.yml

@@ -4,11 +4,14 @@ php:
   - 5.6
   - 7.0
   - hhvm
+  - nightly
 
 matrix:
   allow_failures:
-    - php: hhvm
-
+    - php:
+      - hhvm
+      - nightly
+
 cache:
   directories:
     - $HOME/.composer/cache

src/TypeResolver.php

@@ -38,7 +38,7 @@ final class TypeResolver
         'mixed' => 'phpDocumentor\Reflection\Types\Mixed',
         'array' => 'phpDocumentor\Reflection\Types\Array_',
         'resource' => 'phpDocumentor\Reflection\Types\Resource',
-        'void' => 'phpDocumentor\Reflection\Types\Void',
+        'void' => 'phpDocumentor\Reflection\Types\Void_',
         'null' => 'phpDocumentor\Reflection\Types\Null_',
         'scalar' => 'phpDocumentor\Reflection\Types\Scalar',
         'callback' => 'phpDocumentor\Reflection\Types\Callable_',

src/Types/Void.php → src/Types/Void_.php

@@ -20,7 +20,7 @@
  * Void is generally only used when working with return types as it signifies that the method intentionally does not
  * return any value.
  */
-final class Void implements Type
+final class Void_ implements Type
 {
     /**
      * Returns a rendered output of the Type as it would be used in a DocBlock.

tests/unit/TypeResolverTest.php

@@ -373,7 +373,7 @@ public function provideKeywords()
             ['scalar', 'phpDocumentor\Reflection\Types\Scalar'],
             ['object', 'phpDocumentor\Reflection\Types\Object_'],
             ['mixed', 'phpDocumentor\Reflection\Types\Mixed'],
-            ['void', 'phpDocumentor\Reflection\Types\Void'],
+            ['void', 'phpDocumentor\Reflection\Types\Void_'],
             ['$this', 'phpDocumentor\Reflection\Types\This'],
             ['static', 'phpDocumentor\Reflection\Types\Static_'],
             ['self', 'phpDocumentor\Reflection\Types\Self_'],