Avoid move constructor of std::fstream for GCC < 5 (#3098)

Avoid move constructor of std::fstream for GCC < 5 (#3098)

GCC < 5 does not support the move constructor of `std::fstream` (see
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54316 for details). This
commit wraps the `std::fstream` in an `std::unique_ptr` to work around
that issue.