Fixed compilation error on OSX with GNU 6.2.0 (#59)

`error: 'exp' is not a member of 'std'
     rec[i] = std::exp(rec[i] - wmax);`

include/LightGBM/utils/common.h

@@ -9,6 +9,7 @@
 #include <sstream>
 #include <cstdint>
 #include <algorithm>
+#include <cmath>
 
 namespace LightGBM {