Build docs on nightly (#14)

This is needed for the findcallers docstring

Author: timholy

.travis.yml

@@ -18,7 +18,7 @@ jobs:
   fast_finish: true
   include:
     - stage: Documentation
-      julia: 1
+      julia: nightly
       script: julia --project=docs -e '
           using Pkg;
           Pkg.develop(PackageSpec(path=pwd()));