Various tweaks to fetch-develop-deps

* tell git to checkout the right branch, rather than cloning and then changing.
* clone with depth 1 under travis, to save time.
* less pushd/popd, which print out confusing text - use `cd` in a subshell
  instead. (and just avoid it where possible).
* add some markers to the output to let travis roll it up.

.travis.yml

@@ -13,5 +13,6 @@ node_js:
     - 6
     - 7
 install:
-    - scripts/fetch-develop.deps.sh
-    - npm install
+    # clone the deps with depth 1: we know we will only ever need that one
+    # commit.
+    - scripts/fetch-develop.deps.sh --depth 1 && npm install