fix get the latest release tag error

[kslr]

#2373

[sunshineplan]

https://developer.github.com/v3/#user-agent-required
cURL sends a valid User-Agent header by default. If you provide an invalid User-Agent header via cURL (or via an alternative client), you will receive a 403 Forbidden response:

curl -iH 'User-Agent: ' https://api.github.com/meta
HTTP/1.0 403 Forbidden
Connection: close
Content-Type: text/html
Request forbidden by administrative rules.
Please make sure your request has a User-Agent header.
Check https://developer.github.com for other possible causes.

官方文档的描述，curl会自带默认的User-Agent，所以其实也没必要另外再加个。

[sunshineplan]

https://developer.github.com/v3/#current-version
By default, all requests to https://api.github.com receive the v3 version of the REST API. We encourage you to explicitly request this version via the Accept header.

Accept: application/vnd.github.v3+json

按照推荐使用以下代码测试60次(不进行认证，每个IP每小时只能使用api60次)

#! /bin/bash

for i in {1..60}
do
    curl -si https://api.github.com/repos/v2ray/v2ray-core/releases/latest -H "Accept: application/vnd.github.v3+json" >> test
done

全都能返回正常的json格式，当然使用-H "Accept: application/json"也通过了测试，唯一的区别是在返回头中X-GitHub-Media-Type: github.v3，而不是X-GitHub-Media-Type: github.v3; format=json，这个应该也没太大影响。
根据上面的测试，未发现User-Agent为必须信息(curl自带User-Agent，如curl/7.55.1，并且被Github所接受)，所以建议删除伪装该信息。

[kslr]

我记得我更新了，但看起来好像没有，只能等待下一次了