Fix #10: missing kibana in the docker image

The npm postinstall script was failing when run by the root user during a docker build with the following:
```
npm WARN cannot run in wd [email protected] [ -f kibana/index.html ] && exit 0; mkdir -p kibana; curl https://download.elasticsearch.org/kibana/kibana/kibana-3.1.2.tar.gz | tar xvzf - -C kibana --strip-components=1 (wd=/app)
```
The docker build executes as the root user but npm downgrades its permissions during the npm install.
adding universal read and write permissions to the app folder during the build does not prevent the issue.

Running `npm install --unsafe-perm` solves the problem.

Dockerfile

@@ -3,7 +3,7 @@ MAINTAINER Hugues MALPHETTES <[email protected]>
 
 WORKDIR /app
 ADD . /app
-RUN npm install
+RUN npm install --unsafe-perm
 
 EXPOSE 3003