Update rvm.py

there is a bug in the code that causes _rvm to return False when rvm is not installed on the root account.
this commit passes runas to is_installed so it queries the correct user

i'm a total n00b with python, so please excuse me for not writing a test case for this.  hope it helps.

steps to duplicate the bad behaviour before this patch:

> [root@saltstack]# salt slave02\* rvm.list runas=myuser
    slave02.XXXXXXX:
        Traceback (most recent call last):
          File "/usr/lib/python2.6/site-packages/salt/minion.py", line 418, in _thread_return
            ret['return'] = func(*args, **kwargs)
          File "/usr/lib/python2.6/site-packages/salt/modules/rvm.py", line 118, in list
            for line in _rvm('list', '', runas=runas).splitlines():
        AttributeError: 'bool' object has no attribute 'splitlines'

> #install rvm globally
    [root@saltstack]# salt slave02\* rvm.install
    slave02.XXXXXXX:
    True
    [root@saltstack]# salt slave02\* rvm.list runas=myuser
    slave02.XXXXXXX:

now that it's installed under the root user, _rvm returns a non-boolean