Use make_shared<T> instead of shared_ptr(new T()) in a few places

Summary: It's marginally more efficient, so might as well encourage it.

Reviewed By: @swtaarrs

Differential Revision: D1744798

Signature: t1:1744798:1418813788:a1c2f67eefe87c0a8ef5ecc73031742b0cd40ed0