Use --user for pip, to be compatible with Deps.pip

.travis.yml

@@ -36,7 +36,7 @@ before_script:
   - julia -e 'map(Pkg.add, ["Coverage", "Documenter"])'
   # Install Python packages specified in docs/make.jl here so that
   # they will be cached:
-  - pip install --upgrade mkdocs python-markdown-math mkdocs-alabaster
+  - pip install --user --upgrade mkdocs python-markdown-math mkdocs-alabaster
 
 ## uncomment the following lines to override the default test script
 #script: