Cap Documenter.jl to 0.19 on Travis (#13)

Documenter 0.20 will introduce breaking changes that will invalidate
existing make.jl setups. This commit makes sure that automatic Travis
builds will not use 0.20 automatically, in order to avoid sudden
documentation deployment failures once Documenter 0.20 gets tagged.

This commit has been generated by a script.

.travis.yml

@@ -14,6 +14,6 @@ matrix:
     - julia: nightly
   fast_finish: true
 after_success:
-  - julia -e 'using Pkg; Pkg.add("Documenter"); Pkg.add("Coverage")'
+  - julia -e 'using Pkg; ps=Pkg.PackageSpec(name="Documenter", version="0.19"); Pkg.add(ps); Pkg.pin(ps); Pkg.add("Coverage")'
   - julia -e 'using Coverage; import SubstitutionModels; cd(dirname(dirname(pathof(SubstitutionModels)))); Codecov.submit(process_folder())'
   - julia -e 'import SubstitutionModels; cd(dirname(dirname(pathof(SubstitutionModels)))); include(joinpath("docs", "make.jl"))'