Triangle: equilateral triangles are isosceles

[clemensw]

I agree the coding is more interesting, but I have a hard time accepting
assert!(triangle.is_equilateral());
assert!(!triangle.is_isosceles());

[petertseng]

Given that exercism/problem-specifications#379 has been discussed and exercism/problem-specifications#424 has been merged as a result, we should resolve this issue by complying with the suite.

Re-visiting #206 would be a quick win in this area, though it may be worth it to rewrite the test suite follow the structure of the new JSON file.

[IanWhitney]

Agreeing with @petertseng here. I think if you have an approach that you want to suggest, updating the official JSON file is the way to go.