[composer.json] add vendor name to fix install error

[tetsunosuke]

composer install occurs error.

  [Composer\Json\JsonValidationException]
  "./composer.json" does not match the expected JSON schema:
   - name : Does not match the regex pattern ^[a-z0-9]([_.-]?[a-z0-9]+)*/[a-z
  0-9](([_.]?|-{0,2})[a-z0-9]+)*$

This is caused by missing vendor-name on "name" in composer.json, so add the GitHub username(sendgrid) as a vendor-name.