Fix a bash 4.4 warning when installing jruby
Fix a bash 4.4 warning when installing jruby

In particular, the following:

```
/path/to/bin/ruby-build: line 739: warning: command substitution: ignored null byte in input
```

makandra/ruby-build,dark-panda/ruby-build,ryanfb/ruby-build,dark-panda/ruby-build,rbenv/ruby-build,sstephenson/ruby-build,rbenv/ruby-build