This fixes#511
It allows the virtualenv plugin to call back and tell the airline plugin
to update the statusline. Therefore, the new function
airline#extensions#virtualenv#update() is provided. However, to properly
fix this issue, the virtualenv plugin needs to call this function.
Second, depending on $VIRTUAL_ENV is not safe, since the virtualenv
plugin might define it itsself when calling :VirtualEnvActivate
Therefore skip this test and just test for existance of he
:VirtualEnvList command.
