Add post_install_message to gemspec
Add post_install_message to gemspec

This will remind users to install Overcommit in their git repository
after they have installed the gem.

Change-Id: Ife948c194c1acdc4ae9361ef75d19e4988c5aadc
Reviewed-on: http://gerrit.causes.com/35616
Reviewed-by: Shane da Silva <6f6e68d1df92f30cb4b3ce35260ddf94a402f33d@causes.com>
Tested-by: Shane da Silva <6f6e68d1df92f30cb4b3ce35260ddf94a402f33d@causes.com>

gnagel/overcommit,TheSooth/overcommit,oxocode/overcommit,oxocode/overcommit,blech75/overcommit,gnagel/overcommit,blech75/overcommit,TheSooth/overcommit