fix vertical width

[amhunt]

@mariepw's PR removed the condition that the calendar must be horizontal to apply width styling. This broke vertical calendar widths by forcing them to be the width that they would be if they were horizontal and also by making it inline-block.

Here's the problem line: https://github.com/airbnb/react-dates/pull/989/files#diff-883056b4b11263241c339f927e93411bR818

Also, I removed a style object that didn't exist, DayPicker__vertical.

Here's what the calendar looked like:

Here's what it should look like:

reviewers: @majapw

[majapw]

This seems legit to me!

[coveralls]

Coverage remained the same at 86.513% when pulling f11c74e on andrew-cal-fix into 1cb4dd6 on master.