Update urdfdom.rb

Remove tap in depends_on key.

The reference to the tap is not needed and if anything only a hinderance (e.g. if some installs their custom version of those packages from their own tap). Also, rosdep can't properly resolve the dependency that way (note the warning at the end):

```
$ rosdep install --from-paths src --ignore-src --rosdistro indigo
executing command [brew install urdfdom_headers]
==> Downloading https://github.com/ros/urdfdom_headers/archive/0.2.3.tar.gz
######################################################################## 100,0%
==> cmake . -DCMAKE_INSTALL_PREFIX=/usr/local/Cellar/urdfdom_headers/0.2.3 -DCMAKE_BUILD_TYPE=None -DCMAKE_FIND_FRAMEWORK=LAST -DCMAKE_VERBOSE_MAKEFILE=ON -Wno-dev
==> make install
  /usr/local/Cellar/urdfdom_headers/0.2.3: 16 files, 84K, built in 4 seconds
executing command [brew install urdfdom]
==> Installing urdfdom dependency: ros/deps/console_bridge
==> Downloading https://github.com/ros/console_bridge/archive/0.2.5.tar.gz
######################################################################## 100,0%
==> cmake . -DCMAKE_INSTALL_PREFIX=/usr/local/Cellar/console_bridge/0.2.5 -DCMAKE_BUILD_TYPE=None -DCMAKE_FIND_FRAMEWORK=LAST -DCMAKE_VERBOSE_MAKEFILE=ON -Wno-dev
==> make install
  /usr/local/Cellar/console_bridge/0.2.5: 6 files, 72K, built in 4 seconds
==> Installing urdfdom
==> Downloading https://github.com/ros/urdfdom/archive/0.2.10.tar.gz
######################################################################## 100,0%
==> cmake . -DCMAKE_INSTALL_PREFIX=/usr/local/Cellar/urdfdom/0.2.10 -DCMAKE_BUILD_TYPE=None -DCMAKE_FIND_FRAMEWORK=LAST -DCMAKE_VERBOSE_MAKEFILE=ON -Wno-dev
==> make install
  /usr/local/Cellar/urdfdom/0.2.10: 14 files, 712K, built in 8 seconds
executing command [brew install console_bridge]
Warning: console_bridge-0.2.5 already installed
#All required rosdeps installed successfully
```