fixes the Gemfile generator templates

The templates were written as if for a given gem you could either pass a version
or options, but not both. But you may want to specify a version and also a group
or whether the gem has to be required, for example.
upgrade SDoc

Kudos to @zzak for taking over SDoc and make it work with RDoc 4.
fixes a typo in a CHANGELOG
Skip Spring App Generator tests on JRuby

Spring makes extensive use of Process.fork, so won't be able to provide
JRuby Support, hence skip these tests on JRuby.