chore(ci): use nvmrc & npmrc to set nodejs/npm version/options

.npmrc

@@ -0,0 +1 @@
+save-exact=true

.nvmrc

@@ -0,0 +1 @@
+8.11.4

.travis.yml

@@ -7,9 +7,6 @@ defaults_go: &DEFAULTS_GO
 
 defaults_js: &DEFAULTS_JS
   language: node_js
-  # FIXME 8.12 includes npm 6.4.1 which generates a different lock file on
-  # macOS and Linux, causing diffs on CI and we fail builds when that happens
-  node_js: "8.11.4" # FIXME same in the 'Build and Deploy' stage
   # install defaults to "npm install", which is done via make
   install: []
   cache:
@@ -65,7 +62,7 @@ jobs:
       before_script:
         # this stage needs to build everything including assets file and that
         # requires running webpack, so we need nodejs here
-        - nvm install 8.11.4
+        - nvm install $(< .nvmrc)
       script:
         # compile assets via webpack and build those into bindata_assetfs.go file
         - make bindata_assetfs.go