Fix clang-tidy warnings/errors

[springmeyer]

Since enabling clang-tidy we now have travis job that runs it. To get this travis job passing we'll need to fix these warnings (probably minor, but let's see):

/home/travis/build/mapbox/spatial-algorithms/bench/scaling.cpp:2:1: error: #includes are not sorted properly [llvm-include-order,-warnings-as-errors]
#include <mapbox/geometry.hpp>
^
/home/travis/build/mapbox/spatial-algorithms/bench/scaling.cpp:6:1: error: do not use namespace using-directives; use using-declarations instead [google-build-using-namespace,-warnings-as-errors]
using namespace mapbox::geometry;
^
/home/travis/build/mapbox/spatial-algorithms/bench/scaling.cpp:15:30: error: rand() has limited randomness; use C++11 random library instead [cert-msc30-c,-warnings-as-errors]
        point<double> input{(rand() % 500) / 25.0, (rand() % 500) / 25.0};
                             ^
/home/travis/build/mapbox/spatial-algorithms/bench/scaling.cpp:15:53: error: rand() has limited randomness; use C++11 random library instead [cert-msc30-c,-warnings-as-errors]
        point<double> input{(rand() % 500) / 25.0, (rand() % 500) / 25.0};
                                                    ^

Lots more visible at https://travis-ci.org/mapbox/spatial-algorithms/jobs/339174781#L1021. To see them locally run:

make tidy