Suppress Clang's warning on zero as a null pointer

[feroldi]

This suppresses the Clang warning -Wzero-as-null-pointer-constant by using FMT_NULL instead of 0:

fmt/format.cc:124:24: error: zero as null pointer
                      constant [-Werror,-Wzero-as-null-pointer-constant]
  FMT_ASSERT(buffer != 0 && buffer_size != 0, "invalid buffer");
                       ^
                       nullptr

[feroldi]

It seems there's an error with npm dependencies on Travis environment?

-npm WARN engine less@2.7.2: wanted: {"node":">=0.12"} (current: {"node":"0.10.48","npm":"2.15.1"})

[vitaut]

Merged, thanks!

Something is messed up with npm dependencies, I'll try to install an older version of less.