tsc: adjust TSC membership for IBM+StrongLoop

Following the IBM+StrongLoop announcement last week, we
(@piscisaureus, @bnoordhuis, @srl295, @mhdawson, and myself)
want to take a moment to reiterate our personal commitment
to Node.js and the Foundation.

Per the TSC rules, any one company is limited to no more than
1/4 of the voting seats on the TSC and after talking it over
amongst ourselves, @srl295 and @mhdawson have elected to step
back from the TSC for now.

What I would propose is that they continue to participate in the
weekly TSC meetings as observers; and that once the membership
expands, they be considered once again for full TSC status.

Both Steven and Michael will continue to be actively involved
with Node.js. Both are active members of various Working Groups.
(Michael chairs the Benchmarking workgroup and Steven chairs the
Intl workgroup.)

/cc @nodejs/tsc

Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Fedor Indutny <fedo...