'EXPECT_SOME' and other asserts don't work with 'Try's that have a custom error state.

MESOS-5110 introduced an additional template parameter for Try to support custom error types. Using these values with EXPECT_SOME doesn't work, i.e.

Try<Foo, FooError> foo = bar();
EXPECT_SOME(foo);

won't compile. The other assertions in stout/gtest.hpp are likely affected as well.