Merge pull request #3013 from zzak/dont-pull-gems-by-default

This patch changes git gem behavior to never pull gems unless specified.

tasks/mruby_build_gem.rake

@@ -106,7 +106,7 @@ module MRuby
           git.run_checkout gemdir, params[:checksum_hash]
         else
           # Jump to the top of the branch
-          git.run_checkout gemdir, branch
+          git.run_checkout gemdir, branch if $pull_gems
         end
       else
         fail "unknown gem option #{params}"