Update automatic document generation

It looks document generation proceduce has slightly changed.
docs/Project.toml is defining dependencies for document generation and
they are not explicitly given in `travis.yml`.

.travis.yml

@@ -20,12 +20,10 @@ matrix:
 jobs:
     include:
         - stage: "Documentation"
-          julia: 1.0
           os: linux
           script:
-            - julia --project=docs/ -e 'using Pkg; Pkg.develop(PackageSpec(path=pwd()));
-                                                   Pkg.instantiate()'
-            - julia --project=docs/ -e 'import Pkg; Pkg.add("Documenter"); include("docs/make.jl")'
+            - julia --project=docs/ -e 'using Pkg; Pkg.develop(PackageSpec(path=pwd())); Pkg.instantiate()'
+            - julia --project=docs/ docs/make.jl
           after_success: skip
 
 after_success:

docs/Project.toml

@@ -0,0 +1,3 @@
+[deps]
+Documenter = "e30172f5-a6a5-5a46-863b-614d45cd2de4"
+Literate = "98b081ad-f1c9-55d3-8b20-4c87d4299306"