Update .travis.yml (#211)

Author: mileslucas

.travis.yml

@@ -25,6 +25,8 @@ jobs:
     - stage: "Documentation"
       julia: 1.4
       os: linux
+      env:
+        - GKSwstype="100"
       script:
         - julia --project=docs/ -e 'using Pkg; Pkg.instantiate(); Pkg.develop(PackageSpec(path=pwd()))'
         - julia --project=docs/ docs/make.jl