Allow small ッ before a vowel

This is a nonstandard use of ッ and it doesn't really have a defined
romanization, but we need to support it for Cramorant (ウッウ, U'u).

pokedex/roomaji.py

@@ -98,8 +98,8 @@ class Romanizer(object):
 
                 if last_kana == 'sokuon':
                     if kana[0] in vowels:
-                        raise ValueError("Sokuon cannot precede a vowel.")
-
+                        characters.append("'")
+                    else:
                         characters.append(kana[0])
                 elif last_kana == 'n' and kana[0] in vowels:
                     characters.append("'")

pokedex/tests/test_roomaji.py

@@ -25,6 +25,7 @@ import pokedex.roomaji
         (u'ラティアス',         'ratiasu'),
         (u'ウィー',             'wii'),
         (u'セレビィ',           'sereby'),
+        (u'ウッウ',             u"u'u"),
     ]
 )
 def test_roomaji(kana, roomaji):
@@ -52,6 +53,7 @@ def test_roomaji(kana, roomaji):
         (u'ラティアス',         u'ratiasu'),
         (u'ウィー',             u'wí'),
         (u'セレビィ',           u'serebí'),
+        (u'ウッウ',             u"u'u"),
     ]
 )
 def test_roomaji_cs(kana, roomaji):