Fix PopperJS instantiation (see issue #32)

[patreeceeo]

This fixes issues relating to this._popper being undefined, like #32. My personal motivation for this change is an issue where when the popper overflows the boundaries element on initial render, the position doesn't get fixed until there's a resize event.

I can create an example of this issue if you want. I can also add some tests to this PR.

Thanks for working on this lib, by the way!

[patreeceeo]

So, I put together some code bins replicating my issue.

Here's a code bin showing my issue: https://www.webpackbin.com/bins/-L0-y6SHzbWdBcEn-lVf

Here's a code bin showing the issue fixed by these changes: https://www.webpackbin.com/bins/-L0-yRUS35RxpZS1XYAX

[sontek]

hey @souporserious, any chance this PR could get reviewed?

[souporserious]

Sorry about the wait, I've been busy with work and the holidays. This looks great! Thanks for the examples and fixes, I really appreciate it 😃