Cap Documenter.jl to 0.19 on Travis

[mortenpi]

Hi! This is an automated pull request by @mortenpi.

The next Documenter.jl release (0.20) will make breaking changes to the makedocs and deploydocs functions. This PR makes sure that your Travis CI runs will install and use Documenter 0.19, so that documentation deployment would not break when 0.20 is released.

Subscribe to JuliaDocs/Documenter.jl#861 to be notified about the 0.20 release. The release will come with release notes detailing how to upgrade your make.jl script. Note that the conversation in that issue is locked, so you should not receive any other notifications from there.

If there is a problem with the PR, please tag @mortenpi.

[ExpandingMan]

The documenter hook is currently borken anyway. This might be a good opportunity for us to update it and get the updated version working.