Remove JRuby workaround for Travis CI

Follow up for #4703.

JRuby 9.1.13 and 9.2.0.0-SNAPSHOT (head) are specified for Travis CI.

`Dir.tmpdir` on JRuby 9.1.6 or higher returns `/tmp`. It is not the
current directory.

JRuby 9.1.5 or lower
====================

```console
% cd /home/vagrant && rvm use jruby-9.1.5.0 && ruby -rtmpdir -ve 'p Dir.tmpdir'
Using /home/vagrant/.rvm/gems/jruby-9.1.5.0
jruby 9.1.5.0 (2.3.1) 2016-09-07 036ce39 OpenJDK 64-Bit Server VM
24.121-b00 on 1.7.0_121-b00 +jit [linux-x86_64]
/home/vagrant/.rvm/rubies/jruby-9.1.5.0/lib/ruby/stdlib/tmpdir.rb:40:
warning: shadowing outer local variable - dir
"/home/vagrant"
```

JRuby 9.1.6 or higher
=====================

```console
% cd /home/vagrant && rvm use jruby-9.1.6.0 && ruby -rtmpdir -ve 'p Dir.tmpdir'
Using /home/vagrant/.rvm/gems/jruby-9.1.6.0
jruby 9.1.6.0 (2.3.1) 2016-11-09 0150a76 OpenJDK 64-Bit Server VM
24.121-b00 on 1.7.0_121-b00 +jit [linux-x86_64]
"/tmp"
```

I think that this workaround code is unnecessary already.

Related Issue ... jruby/jruby#405