fix(ci): restore nvmrc, we still need to manage nodejs versions

.nvmrc

@@ -0,0 +1 @@
+8.12.0

.travis.yml

@@ -7,7 +7,6 @@ defaults_go: &DEFAULTS_GO
 
 defaults_js: &DEFAULTS_JS
   language: node_js
-  node_js: "8.12.0"
   # we run make to install everything
   install: []
   cache:
@@ -63,6 +62,10 @@ jobs:
       cache:
         # disable cache so we always build from scratch
         directories: []
+      before_script:
+        # this stage needs to build everything including assets file and that
+        # requires running webpack, so we need nodejs here
+        - nvm install $(< .nvmrc)
       script:
         # compile assets via webpack and build those into bindata_assetfs.go file
         - make bindata_assetfs.go