.travis.yml: DISPLAY=true for 8.4

.travis.yml

@@ -161,7 +161,7 @@ matrix:
               # Images are rendered differently on different systems, so filter them out in the comparison
               diff <(grep -v image/png ~/ihaskell-out.ipynb) <(grep -v image/png notebooks/IHaskell.ipynb)
           fi
-    - env: DISPLAY=false   USE_STACK_YAML="stack-8.4.yaml"     # GHC 8.4.1
+    - env: DISPLAY=true   USE_STACK_YAML="stack-8.4.yaml"     # GHC 8.4.3
       language: python
       # Test against different stack LTS versions.