devel/RStudio: unbreak with boost 1.67

src/cpp/core/DateTime.cpp:68:26: error: no matching conversion for functional-style cast from 'double' to 'boost::posix_time::seconds'

return time_t_epoch + seconds(sec);
                      ^~~~~~~~~~~

/usr/local/include/boost/date_time/posix_time/posix_time_duration.hpp:53:30: note: candidate constructor (the implicit copy constructor) not viable: no known conversion from 'double' to 'const boost::posix_time::seconds' for 1st argument

class BOOST_SYMBOL_VISIBLE seconds : public time_duration
                           ^

/usr/local/include/boost/date_time/posix_time/posix_time_duration.hpp:58:37: note: candidate template ignored: disabled by 'enable_if' [with T = double]

typename boost::enable_if<boost::is_integral<T>, void>::type* = 0) :
                          ^

PR: 227427
Reported by: antoine (via exp-run)