Drop support for GCC < 4.8 in folly/test/FBStringTest.cpp

Summary: [Folly] Drop support for GCC < 4.8 in folly/test/FBStringTest.cpp.

Can't stay indefinitely backcompat with older compilers.

Reviewed By: @tudor

Differential Revision: D2422425

folly/test/FBStringTest.cpp

@@ -1291,15 +1291,12 @@ TEST(FBString, testFrontBack) {
 
 TEST(FBString, noexcept) {
   EXPECT_TRUE(noexcept(fbstring()));
-  // std::move is not marked noexcept in gcc 4.6, sigh
-#if __GNUC_PREREQ(4, 7)
   fbstring x;
   EXPECT_FALSE(noexcept(fbstring(x)));
   EXPECT_TRUE(noexcept(fbstring(std::move(x))));
   fbstring y;
   EXPECT_FALSE(noexcept(y = x));
   EXPECT_TRUE(noexcept(y = std::move(x)));
-#endif
 }
 
 TEST(FBString, iomanip) {