Only allow dict serialization if Boost version is >= 1.56
Only allow dict serialization if Boost version is >= 1.56


Former-commit-id: ad436a0867b7fe3c5d0e637cbeb917bc93b4a409
xunzhang/dynet,danielhers/dynet,clab/dynet,danielhers/dynet,clab/dynet,chunyang-wen/dynet,chunyang-wen/dynet,shuheik/dynet,danielhers/dynet,clab/dynet,danielhers/dynet,shuheik/dynet,xunzhang/dynet,shuheik/dynet,clab/cnn,clab/cnn,xunzhang/dynet,chunyang-wen/dynet,danielhers/dynet,xunzhang/dynet,shuheik/dynet,shuheik/dynet,clab/cnn,clab/dynet,clab/dynet,danielhers/dynet,clab/dynet,shuheik/dynet,shuheik/dynet,chunyang-wen/dynet,danielhers/dynet,clab/dynet