Pass default version into gem_package

Without it, gem_package will pass an empty version string to "gem install" which will fail.

➜ root@bao-test ~  /usr/bin/gem install ruby-shadow -q --no-rdoc --no-ri -v ""
ERROR:  While executing gem ... (ArgumentError)
    Illformed requirement [""]
➜ root@bao-test ~  /usr/bin/gem install ruby-shadow -q --no-rdoc --no-ri -v ">=0"
Building native extensions.  This could take a while...
Successfully installed ruby-shadow-2.3.3
1 gem installed