fix dropdown menu position for IE 11

[jharris4]

I just noticed that when using react-select in IE 11 the dropdown menu was incorrectly showing up to the right of the input/value. This may be related to the fact that I have select component nested inside a flex layout.

Adding the left: 0 style fixed it for me so I figured I'd make a PR in case anyone else hits the same issue.

I tried to add a screenshot, but it closes the dropdown as soon as i click to take the screenshot... lol

[coveralls]

Coverage remained the same at 94.973% when pulling 3f90993 on jharris4:master into 851d94b on JedWatson:master.

[JedWatson]

Thanks @jharris4, I suspect the issue is related to the nesting or it should have shown up earlier, but this fix doesn't seem to have any other side-effects so it looks good to me.