[README] Use `application/json`

The MIME media type for JSON text is application/json. The default encoding is UTF-8. (Source: RFC 4627).
Merge pull request #75 from kylef/patch-1

[README] Use `application/json`
[CHANGELOG] Fix an error in version numbers (thx @zygoat)
[3.1.8] Fixed crash when loading an empty fixture file.