Upgrade sendgrid/php-http-client

[akeeman]

Version 3.2.0 and above from the sendgrid/php-http-client library use improved autoloading, fixing a problem where \SendGrid\Response cannot be found when using a non-optimized composer installation.
When semantic versioning is applied, it is save to upgrade to newer feature releases.

[akeeman]

I sent the cla to [email protected].

[thinkingserious]

Thanks for the pull request @akeeman!

Do you want to try solving the failed tests or would you like us to take this one over?

[akeeman]

As a specific version of the php-http-client is used, it works for now to use reflection properties to check the host, request headers and version of the client.
The client should provide getters for these. I have provided a pr there to fix this. Maybe it's wise to check if that one will be accepted first. Winks to @thinkingserious
It would be nice for the php-http-client to use semantic versioning so the latest bugs fixes and features can be used here too without too much hassle. I opened an issue for that here.

[thinkingserious]

@akeeman,

I have merged in your PR, could you please update this pull request?

Thanks!

[akeeman]

There you go!

You indicated that the php-http-client library should follow semver standards. Therefore I set the php-http-client version back to to "~3.4", instead of "3.4" (note the tilde!).