Update the .travis.yml so that the correct Python and matplotlib versions are installed when Travis CI builds the documentation. This should fix issues with PyCall being able to call down to Matplotlib to build plots for the site.

Author: kernelmethod

.travis.yml

@@ -62,10 +62,16 @@ jobs:
     ### Documentation generation
     - stage: "Documentation"
       julia: 1.3
-      python: 3.7
       os: linux
+      dist: bionic
+      install:
+        - sudo apt-get update
+        - sudo apt-get install -y python3.7 python3-pip python3-setuptools
+        - pip3 install --upgrade pip
+        - pip3 install --user matplotlib
+        - julia --project=docs -e 'using Pkg; Pkg.instantiate(); \
+                                   Pkg.add(PackageSpec(path=pwd()));'
+        - julia --project=docs -e 'using Pkg; Pkg.build("PyCall");'
       script:
-        - pip install --user matplotlib
-        - julia --project=docs -e 'using Pkg; Pkg.instantiate(); Pkg.add(PackageSpec(path=pwd()))'
         - julia --project=docs --color=yes docs/make.jl
       after_success: skip