Adding documenter step

Author: GearsAD

.travis.yml

@@ -21,7 +21,7 @@ jobs:
       julia: 1.0
       os: linux
       script:
-        - julia -e 'import Pkg; Pkg.add("Documenter")'
+        - julia -e 'import Pkg; Pkg.add("Documenter"); Pkg.add("DataFrames")'
         - julia --project=docs/ -e 'using Pkg; Pkg.develop(PackageSpec(path=pwd()));
                                                Pkg.instantiate()'
         - julia --project=docs/ docs/make.jl