Upgrade Weave Net to latest version of build-tools, skipping bash scripts when lint-ing.

Lint-ing all of Weave Net is currently not desirable because:

* it would create conflicts on many branches,
* it would pollute the git history,
* etc.

and because `lint` currently lints all files except `./vendor`, this breaks Weave Net's build.
As a result, Weave Net is not using the latest commit in `build-tools`'s `master` branch, which is also source of problems:

* it limits standardisation of projects across Weaveworks,
* it limits the ability to use the latest features of `build-tools`,
* it complexifies merging changes in `build-tools` (e.g. #53 and weaveworks/weave#2694 for weaveworks/weave#2647),
* etc.

This change:

* brings Weave Net to the latest version of `build-tools` (which now supports `.lintignore` files),
* adds such a `.lintignore` file to skip all bash scripts (in order to avoid breaking the build).

`GLOB`s in `.lintignore` can then, over time and as scripts are `shfmt`-ed, be relaxed and maybe even removed.

.lintignore

@@ -0,0 +1,8 @@
+# IMPORTANT: Please only use GLOBs to ignore files and directories.
+
+# Do not lint any bash script in Weave Net, in order to 
+# avoid breaking the build on make lint:
+*.sh
+bin/*
+prog/weaveexec/symlink
+weave