unit tests don't build on CentOS 7

https://jenkins.aws.isc.org/job/kea-dev/job/ut-basic/328/

01:35:14  context_unittest.cc: In member function ‘void {anonymous}::EvalContextTest::checkTokenIntToText(const TokenPtr&, const string&)’:
01:35:15  context_unittest.cc:517:30: error: ‘boost::core’ has not been declared
01:35:15                      << boost::core::demangle(typeid(n).name()) << " error: "
01:35:15                                ^
01:35:15  context_unittest.cc: In instantiation of ‘void {anonymous}::EvalContextTest::checkTokenIntToText(const TokenPtr&, const string&) [with IntegerType = signed char; TokenIntegerType = isc::dhcp::TokenInt8ToText; isc::dhcp::TokenPtr = boost::shared_ptr<isc::dhcp::Token>; std::string = std::basic_string<char>]’:

CentOS 7 has boost 1.53 by default