Adds a symlink to gradle binary in /usr/bin

This makes gradle instantly accesible to jenkins.
Having both certainly adds two gradle executables to the system.  

A better alternative would be to edit the /etc/profile.d/gradle.sh remove the following line
```
- export PATH=$PATH:$GRADLE_HOME/bin
```
and just use it to set $GRADLE_HOME

```
export GRADLE_HOME={{ gradle_installation_directory }}
```