Don't use NODE_ENV=production for the matrix

This is something we want enabled for all builds, so it should be a global. This should simplify the matrix. The builds without an NPM_SCRIPT didn't do anything anyway.

.travis.yml

@@ -3,9 +3,11 @@ node_js:
 - 8
 - node
 env:
+  global:
+  - NODE_ENV=production
+  matrix:
   - NPM_SCRIPT="tap:unit -- --jobs=4"
   - NPM_SCRIPT="tap:integration -- --jobs=4"
-  - NODE_ENV=production
 sudo: false
 cache:
   directories: