Allow Request to be extended by changing class name 'Request' to static

[bluehaoran]

This replaces #231 because that became a bit of a tangled mess.

This is cleaner and changes just one file, and doesn't fail CI.

[bluehaoran]

CI fails :/

• Does HHVM have issues with the static keyword?
• Is the PHP7 test failing across the board?

[nategood]

I don't believe we've tried testing either. Will add 7 to CI in a different PR.