Getting ready for 0.6

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

v0.11.1

Fixes:

• Multinomial now works with 0 trials
• BetaBinomial accepts larger n
• Documentation fixes for Gamma and Logistic
• Undefined distributions are no longer exported
• Macro escaping on 0.6

New features:

• DirichletMultinomial distribution