[boost] fix in boost/gil/channel_algorithm.hpp for clang 3.9.1

Hi, When compiling with clang 3.9.1, I get an error: /softs/osx32-clang-3.9.1/release/boost/include/boost/gil/extension/io/../../channel_algorithm.hpp:54:85: error: non-type template argument evaluates to -1, which cannot be narrowed to type 'unsigned long long' [-Wc++11-narrowing] struct unsigned_integral_max_value : public mpl::integral_c<UnsignedIntegralChannel,-1> {}; Patch is attached. Frédéric