Disable externalwrench plugin if Gazebo version < 8.0

[gabrielenava]

This fix allow to compile the repo with Gazebo 7. Tested on Ubuntu 16.04 LTS.

[traversaro]

Can you add an ## [Unreleased] section in https://github.com/robotology/gazebo-yarp-plugins/blob/master/CHANGELOG.md with a Fixed subsection that contains a description and link to this PR? Something like:

## [Unreleased]
### Fixed 
* Fixed compilation on Gazebo 7 by disabling compilation of `externalwrench` plugin if the project is configured with Gazebo 7 (https://github.com/robotology/gazebo-yarp-plugins/pull/434) . 

note that in this project we follow the changelog format discussed in https://keepachangelog.com/en/1.0.0/ .

[gabrielenava]

changelog updated.

[traversaro]

Sorry for bothering, but according to the format described in https://keepachangelog.com/en/1.0.0/, the latest change (including the unreleased one) need to go on the top of the document.

[gabrielenava]

changelog fixed

[traversaro]

Thanks!