chore(travis): use bundled npm for node 8/9

There is a bug in Node version 9 that means one has to use the bundled
npm version, as it includes some compatibility patch to keep it working
with Node version 9. Node version 8 and 9 come with a reasonable version
of npm bundled (8.9.0 now has npm 5.5.1 bundled), while versions 7 and
below have older npms, as such this change ensures that when running
node 8 or 9, to use the bundled npm, and for older versions manually
install npm@5 before executing

Author: keithamus

.travis.yml

@@ -9,9 +9,6 @@ cache:
     - node_modules
     - ~/.npm
 
-before_install:
-  - npm i -g npm@5
-
 node_js: 8
 
 jobs:
@@ -46,12 +43,15 @@ jobs:
       script: npm run test:node
     - stage: test
       node_js: 7 # to be removed 2017-06-30
+      before_install: npm i -g npm@5
       script: npm run test:node
     - stage: test
       node_js: 6 # to be removed 2019-04-01
+      before_install: npm i -g npm@5
       script: npm run test:node
     - stage: test
       node_js: 4 # to be removed 2018-04-01
+      before_install: npm i -g npm@5
       script: npm run test:node
     - stage: test
       node_js: lts/* # safety net; don't remove