Travis: run regression tests only in 2.81 exporting

This repo only has reference scenes exported from Blender 2.81.
Therefore, only run smoke test for 2.80 exports and run full
regression tests for 2.81 exports.

.travis.yml

@@ -10,9 +10,9 @@ cache:
 matrix:
     include:
         - name: Blender 2.80
-          env: BLENDER_VERSION=2.80
+          env: BLENDER_VERSION=2.80 MAKE_OPTION=export-blends
         - name: Blender 2.81
-          env: BLENDER_VERSION=2.81
+          env: BLENDER_VERSION=2.81 MAKE_OPTION=all
 
 before_install:
     - sudo apt-get update
@@ -26,4 +26,4 @@ install:
     - source .envs
 
 script:
-    - make all BLENDER=$BLENDER_BIN PYLINT='python3 -m pylint'
+    - make ${MAKE_OPTION} BLENDER=$BLENDER_BIN PYLINT='python3 -m pylint'