[jrubyscripting] check if gems are installed before installing them (#13151)

[jrubyscripting] check if gems are installed before installing them (#13151)

* [jrubyscripting] check if gems are installed before installing them

this avoids unecessary downloads and overwrites, or even more importantly
errors if the GEM_HOME is not writable by the openhab user.

this also adds a configuration option for if you want to check for
updates every time configuration is applied (or openhab restarts). this
is useful if your OpenHAB system can't access the internet regularly.

Signed-off-by: Cody Cutrer <cody@cutrer.us>