Update .ci.base.ruby.yml

mask error if which jruby fails because jruby is not installed.
This which should not even be evaluated but somehow does on DotCi.

```
$ which jruby
/usr/bin/which: no jruby in (/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/bin:/sbin:/home/victorv/bin)

$ [ ! -z "" ] && echo "shouldn't even evaluate past this point" && ln -snf `which jruby` bin/ruby

$ echo $?
1
```