Describe the bug
Unit conversions which divide the underlying value by some integer are accepted without complaint for integer reps.

To Reproduce
Steps to reproduce the behavior:

SI::length_t<int, std::ratio<1>> x{3};
SI::length_t<int, std::ratio<12>> y = x;
std::cout << "x = " << x << "; y = " << y << "!\n";

For me, this prints:

x = 3 m; y = 0 m!

Expected behavior
This should be a compiler error. If the rep is integral, we should only permit conversions which multiply by an integer, since these will stay in the domain of the integers.

Screenshots
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Desktop (please complete the following information):

• OS: Linux
• Browser: chrome
• Version: 22.04

Smartphone (please complete the following information):
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Additional context
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