Fix Japanese translation (#254)

Hello! I made some corrections because I felt the Japanese translation
was strange. It is not common in Japan to write the month in hiragana.
It is more typical to write it using numbers and kanji, like “1月”
(January).

Even In PHP’s Carbon library, the month is also written with numbers and
kanji, like “1月”

https://github.com/briannesbitt/Carbon/blob/master/src/Carbon/Lang/ja.php#L86

lang/jp.json

@@ -1,7 +1,7 @@
 {
 	"name": "Japanese",
-	"months": "いちがつ|にがつ|さんがつ|しがつ|ごがつ|ろくがつ|しちがつ|はちがつ|くがつ|じゅうがつ|じゅういちがつ|じゅうにがつ",
-	"short_months": "1がつ|2がつ|3がつ|4がつ|5がつ|6がつ|7がつ|8がつ|9がつ|10がつ|11がつ|12がつ",
+	"months": "1月|2月|3月|4月|5月|6月|7月|8月|9月|10月|11月|12月",
+	"short_months": "1月|2月|3月|4月|5月|6月|7月|8月|9月|10月|11月|12月",
 	"weeks": "日曜日|月曜日|火曜日|水曜日|木曜日|金曜日|土曜日",
 	"short_weeks": "日|月|火|水|木|金|土",
 	"seasons": "春|夏|秋|冬",