gazebo 6.0 conflict with gazebo 2.2.6(came with ROS indigo)

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When I finished installing ROS Indigo, I noticed that it already came with Gazebo(version2.2.6).

I tried to directly install the 6.0 Gazebo,

but, it was not possible because it looks like 6.0 version and 2.2.6 version conflict with each other.

Actually, I have tried to install gazebo as described in

http://gazebosim.org/tutorials?tut=in...

but, it says that : E: Unable to correct problems, you have held broken package

It looks like that I have Gazebo(version2.2.6) cause this problem

(gazebo which command indicate that I have Gazebo(version2.2.6)

How can I remove Gazebo(version2.2.6) came with ROS indigo?

I don't know how to remove Gazebo(version2.2.6) and install gazebo > 6.0 in order to

run make posix_sitl_default gazebo as described in http://dev.px4.io/simulation-gazebo.html

My situation is as follows:

wkyoun@wkyoun-XPS-13-9343:~$ sudo apt-get install gazebo6

[sudo] password for wkyoun: 
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 gazebo6 : Depends: libgazebo6 (= 6.5.1-1~trusty) but it is not going to be installed
           Depends: libsdformat3 but it is not going to be installed
           Depends: gazebo6-common (= 6.5.1-1~trusty) but it is not going to be installed
           Recommends: gazebo6-plugin-base
E: Unable to correct problems, you have held broken packages.

Also, I tried to run Gazebo as described in

http://dev.px4.io/simulation-gazebo.html

but the error is found:

wkyoun@wkyoun-XPS-13-9343:~/Firmware$ make posix_sitl_default gazebo

 make -C /home/wkyoun/Firmware/build_posix_sitl_default -j4 --no-print-directory gazebo
[  2%] Built target df_driver_framework
[  3%] Built target git_matrix
[  3%] Built target git_genmsg
[  4%] Built target git_gencpp
[  4%] Built target prebuild_targets
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[  5%] Built target generate_topic_listener
[  6%] Built target xml_gen
[  6%] Built target git_driverframework
[  6%] Built target mixer_gen
[  7%] [  7%] Built target msg_gen
Built target git_jmavsim
[  7%] Built target git_gazebo
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[  7%] Built target platforms__common
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[  8%] Built target drivers__pwm_out_sim
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