gem module: support prerelease (--pre)

Adds support for installing prerelease rubygems (--pre flag) to the gem module.  Didn't see preexisting tests but happy to modify one if you point me to it.  I tested this by running it:

ansible all -vvvv -i ../goodeggs-ansible/hosts/local -m gem -c local -a "name=scout prerelease=yes user_install=no state=present"

# gem list
[snip]
scout (5.8.0.pre)
[snip]

fixes https://github.com/ansible/ansible/issues/4393