[BUG] Tests randomly failing

Tests seem to randomly fail with the following error : `Some of your tests did a full page reload`.

This seems to be because of the test `Should have the correct min/max values` in [scale.financialLinear.tests.js](https://github.com/chartjs/chartjs-chart-financial/blob/master/test/specs/scale.financialLinear.tests.js#L9)

* local x1 : 6 ok : 4 fail
* local x1000 : 9 ok ; 1 fail
* local commented : 10 ok ; 0 fail

x1000 means the test is run 1000 times in a for loop, [like this](https://github.com/loicbourgois/chartjs-chart-financial/commit/3452eb2b06fadce703e36238a4a119155880f03c)
commented means the test is commented

And then I tried to run the tests on Travis. The results seem random.
https://travis-ci.org/loicbourgois/chartjs-chart-financial/builds
For each build, node 6, 8, 9 are used.
Usually it works on one of the environment.