try my fix for stan-dev/math log(int) on g++6?

[Bob Carpenter]

I tried to fix the problem with log(int) reported
by ben in stan-dev/math issue #327 on this
branch:

bugfix/0327-log-int

I made sure it didn't break the existing build on
clang++, but I can't test if it fixed the problem, because
I can't get g++6 installed. So if it's easier to just
test the fix than help me install g++6, if it works as
intended above, I can fix it for all the built-in cmath
functions.

And by the way, I thought the issue was going to be
with vectorization, but I don't think that's it, as I
commented on the issue --- at least I couldn't find any
specializations other than the general one for vectorization,
so it looks like Rayleigh did this the right way.

- Bob