Fix user-defined literal tests for MSVC

The test currently tests not only the compiler's support for user-defined literals, but also its constexpr support, which is only partly implemented in MSVC 14. This change removes the constexpr's that MSVC doesn't like so that the tests pass on MSVC 14 CTP2.
Merge pull request #29 from MarcelRaad/patch-1

Fix user-defined literal tests for MSVC