Revert "test 4 and 6 in one go"

This reverts commit 038391ec43.

.travis.yml

@@ -1,8 +1,10 @@
 language: node_js
 node_js:
+  - "4"
+  - "6"
   - "node"
 env:
   global:
     - BUILD_TIMEOUT=10000
 install: npm install
-after_success: sh ./test/ci.sh
+after_success: npm run codecov

test/ci.sh

@@ -1,7 +0,0 @@
-nvm install 6
-npm test
-
-nvm install 4
-npm test
-
-npm run codecov