use versioncmp to compare versions

Using builtin function to compare versions in the watch define (https://docs.puppetlabs.com/references/latest/function.html#versioncmp)
I had to rewrite the related tests as well as they were always testing on the '0.4.1' ```consul::version```. I used mocking on hiera to specify the needed version.
Thank you for taking care of this nice module !