Run visual tests on CI (#3922)

* echo VDIFFR_RUN_TESTS

* Make sure VDIFFR_RUN_TESTS=true on r-release

* remove the echo

Author: cpsievert

.travis.yml

@@ -8,8 +8,9 @@ matrix:
     # avoind errors due to world clock API (hopefully can be removed in future)
     env: _R_CHECK_SYSTEM_CLOCK_=false
   - r: release
-    env: VDIFFR_RUN_TESTS=true
-    env: VDIFFR_LOG_PATH="../vdiffr.Rout.fail"
+    env:
+      - VDIFFR_RUN_TESTS=true
+      - VDIFFR_LOG_PATH="../vdiffr.Rout.fail"
   - r: 3.5
   - r: 3.4
   - r: 3.3