Fix truncation of value warning

[mfigurnov]

Compilation of Caffe on Yosemite with Clang raises the following warning:

CXX src/caffe/common.cpp
src/caffe/common.cpp:28:10: warning: absolute value function 'abs' given an argument of type 'long long' but has parameter of type 'int' which may cause truncation of value [-Wabsolute-value]
  seed = abs(((s * 181) * ((pid - 83) * 359)) % 104729);
         ^
src/caffe/common.cpp:28:10: note: use function 'std::abs' instead
  seed = abs(((s * 181) * ((pid - 83) * 359)) % 104729);
         ^~~
         std::abs
1 warning generated.

This warning is obviously a false positive, since the argument is within the range of int due to the modulo operation. However, for peace of mind, I fixed the warning by replacing abs with std::abs.