update travis for docs'

.travis.yml

@@ -14,5 +14,15 @@ notifications:
     - sunbergzach@gmail.com
 
 after_success:
-- julia --project -e 'import Pkg; ps=Pkg.PackageSpec(name="Documenter", version="0.19"); Pkg.add(ps); Pkg.pin(ps); include("docs/make.jl")'
 - julia --project -e 'import Pkg; Pkg.add("Coverage"); using Coverage; Coveralls.submit(Coveralls.process_folder())'
+
+jobs:
+  include:
+    - stage: "Documentation"
+      julia: 1
+      os: linux
+      script:
+        - julia --project=docs/ -e 'using Pkg; Pkg.develop(PackageSpec(path=pwd()));
+                                               Pkg.instantiate()'
+        - julia --project=docs/ docs/make.jl
+      after_success: skip