run travis builds on oldrel/release/devel, only codecov on release

.travis.yml

@@ -8,8 +8,18 @@ r_check_args: "--as-cran"
 r_github_packages:
   - jimhester/covr
 
+matrix:
+  include:
+    - os: linux
+      r: oldrel
+    - os: linux
+      r: release
+      env: R_CODECOV=true
+    - os: linux
+      r: devel
+
 after_success:
-  - Rscript -e 'covr::codecov()'
+  - if [[ "${R_CODECOV}" ]]; then R -e 'covr::codecov()'; fi
 
 notifications:
   slack: