Fix npm install instructions

The instructions to `npm install grunt --save-dev` and so on for each dependency
are incorrect (even though it works).

That command is for installing a dependency (locally, inside
`./node_modules`) and (with the `--save-dev` flag) writing that dependency to the
`package.json` file.  So this can be used the first time you add a new dependency,
but isn't meant to be used again every time.

Because the dependencies are listed in `package.json` all you need to do to install
them is `npm install`.  You also never need admin rights to do this because it
installs them to `./node_modules` (npm will only install globally with the -g flag,
which is only meant for installing executables, not project dependencies). :)

README.md

@@ -256,15 +256,10 @@ To see how built-in validators are developed, you can look at their sources loca
 
 BootstrapValidator uses [grunt](http://gruntjs.com) to simplify building process.
 
-From the BootstrapValidator root directory, execute the following commands to install the dependent packages (the administrator permission might be required):
+From the BootstrapValidator root directory, install dependencies:
 
 ```bash
-$ npm install grunt --save-dev
-$ npm install grunt-contrib-concat --save-dev
-$ npm install grunt-contrib-copy --save-dev
-$ npm install grunt-contrib-cssmin --save-dev
-$ npm install grunt-contrib-uglify --save-dev
-$ npm install grunt-contrib-watch --save-dev
+$ npm install
 ```
 
 Then, uses grunt to build:
@@ -319,4 +314,4 @@ FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
 COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
 IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
 CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
-```
+```