Use knapsack to parallelize tests

...in order to reduce TravisCI runtimes

.travis.yml

@@ -26,7 +26,14 @@ before_script:
 script:
   - "./travis_script.sh $TEST_SUITE"
 env:
-  - TEST_SUITE=rspec
-  - TEST_SUITE=linters
+  global:
+    - CI_NODE_TOTAL=4
+    - TEST_SUITE=rspec
+  matrix:
+    - TEST_SUITE=linters
+    - CI_NODE_INDEX=0
+    - CI_NODE_INDEX=1
+    - CI_NODE_INDEX=2
+    - CI_NODE_INDEX=3
 matrix:
   fast_finish: true