Update .travis.yml

Author: JeffreySarnoff

.travis.yml

@@ -20,7 +20,7 @@ after_success:
 jobs:
   include:
     - stage: "Documentation"
-      julia: 1.0
+      julia: 1.5.1
       os: linux
       script:
         - julia --project=docs/ -e 'using Pkg; Pkg.instantiate(); Pkg.develop(PackageSpec(path=pwd()))'