Update httplug and php-http/client-common dependencies

[karptonite]

Change Summary

This pull request just updates a couple of seemingly conflicting dependencies which were preventing the package from being installed (see #12). However, I haven't actually had a chance to test if the updates break anything in the package! All I know is that this solves the installation problem. Before merging this, the maintainers should see if this causes any issues for the package.

PR Checklist

• [x] I have read and signed the Contributor License Agreement.

[jasonbosco]

Thanks @karptonite!