Use rbenv to force a decent version of Ruby 1.9.3

Ruby 1.9.3 has a bug in [Psych](https://bugs.ruby-lang.org/issues/6487)

This manifests itself on the default version of ruby available as
`/usr/bin/ruby1.9.1` on our CI machines. Compare:

```
jenkins@ci-slave-2:~/workspace/vcloud-core-broken$ ruby -e "require 'psych' ; puts Psych.load_file '/dev/null'"
/usr/lib/ruby/1.9.1/psych.rb:297:in `initialize': no implicit conversionfrom nil to integer (TypeError)
    from /usr/lib/ruby/1.9.1/psych.rb:297:in `open'
    from /usr/lib/ruby/1.9.1/psych.rb:297:in `load_file'
    from -e:1:in `<main>''
```

```
[gds/vcloud-core:broken-api-token●]$ ruby -e "require 'psych' ; puts Psych.load_file '/dev/null'"
false
```

We hit this bug in our CI tests as described in the [pivotal
story](https://www.pivotaltracker.com/story/show/75357174)

By setting `RBENV_VERSION=1.9.3` in `jenkins_tests.sh` we make our tests
run on a ruby managed by rbenv, which will default to 1.9.3-p545 which does
not have this bug.

However we also need to rebuild the cached gems in bundler, because they
have been built with a shebang of ruby1.9.1 which is not managed by
rbenv so will always be the system ruby. In order to do that nicely,
I've checked in `jenkins_tests.sh` if there are any binstubs with
ruby1.9.1 in them and if so, wiped the gem cache, which will slow down
the first build. Hopefully the addition of `--shebang ruby` to the
bundle install will do the right thing and it will only be the first
build that is slow.

Once this is merged, we can then revert the revert done in #99 and the
new test will hopefully work.