fix 4.9 build

Summary:
see https://www.facebook.com/groups/738749156173702/permalink/798168656898418/

I suppressed "comparison between signed and unsigned integer expressions" warnings for this first pass because there a whole lot (dozens) of those. Do we suppress those with 4.8 or something? Never seen the warnings before.

I can't find anything in boost 1.57 docs/changelog indicating why implicit conversion of boost::optional<bool> -> bool doesn't compile anymore, but it doesn't.

Test Plan: fbconfig -r --platform-all gcc-4.9-glibc-2.20 --extra-cxxflags="-Wno-sign-compare" folly && fbmake opt && fbmake runtests_opt

Reviewed By: rhysparry@fb.com

Subscribers: trunkagent, fugalh, folly-diffs@

FB internal diff: D1765093

Signature: t1:1765093:1420500411:062acf841641c7f4142c9b48e8daa45327de6822

folly/Foreach.h

@@ -48,6 +48,7 @@
  */
 
 #include <boost/type_traits/remove_cv.hpp>
+#include <type_traits>
 
 /*
  * Shorthand for:

folly/test/OptionalTest.cpp

@@ -341,7 +341,7 @@ TEST(Optional, Comparisons) {
   EXPECT_TRUE(6 >  boi);
 
   boost::optional<bool> bob(false);
-  EXPECT_TRUE(bob);
+  EXPECT_TRUE((bool)bob);
   EXPECT_TRUE(bob == false); // well that was confusing
   EXPECT_FALSE(bob != false);
 }

folly/wangle/futures/Promise.h

@@ -17,6 +17,7 @@
 #pragma once
 
 #include <folly/wangle/futures/Try.h>
+#include <functional>
 
 namespace folly { namespace wangle {