shut up, bothering bower!

bower asks for permission to send usage reports, which disturbs the installation process
```
[?] May bower anonymously report usage statistics to improve the tool over time? No
```
tell him to shut up with this

also use npm's bin command to get the local bin folder https://www.npmjs.org/doc/cli/npm-bin.html
@linki looks like we'll need to provide the `--dev` flag to `npm install` to make this work:

```diff
diff --git a/.travis.yml b/.travis.yml
index 306426a..b019453 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -56,7 +56,7 @@ before_install:
   - "nvm use 0.10"
   - "travis_retry npm -g install bower@~1.3.8"
   - "travis_retry bower install -F"
-  - "if [[ $TEST_SUITE == 'karma' ]]; then travis_retry npm install; fi"
+  - "if [[ $TEST_SUITE == 'karma' ]]; then travis_retry npm install --dev; fi"
 before_script:
   - "if [[ $TEST_SUITE != 'karma' ]]; then RAILS_ENV=production bundle exec rake ci:travis:prepare; fi"
 notifications:
```