Update to Julia 0.6 (#570)
* Fix Julia 0.6 deprecations
* Remove promote_eltype and convert_eltype definitions
* Adjust tolerance in multivariate mixture test
* Add QuadGK as dependency
* Delete tests that hook into internal type fields
* Make multivariate and matrix distributions work just with Base's promote mechanisms
* Don't fail if ambiguous methods are detected. Just show them.
* Drop Julia 0.4 support and bump PDMats version
* Use isapprox istead of ≈ to make tests pass on Julia 0.5
* Stop testing on Julia 0.4
