Workaround issue constructing logical vectors of length 1 from 'false'

[jimhester]

Fixes #187

[jimhester]

@sbearrows this seems to be sufficient to fix #187. I removed some existing initializer_list<> overloads that no longer seem necessary, as r_string objects can be constructed from const char* nad std::string. These overloads were causing the problem with g++ 5 as the compiler was either picking these instead of the r_bool one for false for some reason (possibly a compiler bug).

In any case removing these overloads seems to resolve this and the constructor we want is should now be found.

[sbearrows]

Looks good!