CI: revert changes for clang-3.x and revert to using trusty,

clang-3.x seems to have too many issues with xenial.

include/boost/config/compiler/clang.hpp

@@ -269,7 +269,7 @@
 #  define BOOST_NO_CXX14_INITIALIZED_LAMBDA_CAPTURES
 #endif
 
-#if !__has_feature(__cxx_generic_lambdas__) || (__clang_major__ < 4)
+#if !__has_feature(__cxx_generic_lambdas__)
 #  define BOOST_NO_CXX14_GENERIC_LAMBDAS
 #endif
 
@@ -291,7 +291,7 @@
 #  define BOOST_NO_CXX14_CONSTEXPR
 #endif
 
-#if !__has_feature(__cxx_return_type_deduction__) || (__clang_major__ < 4)
+#if !__has_feature(__cxx_return_type_deduction__)
 #  define BOOST_NO_CXX14_RETURN_TYPE_DEDUCTION
 #endif