fix(select): Fixing bug with select menu z-index

[lukebergen]

Some elements were showing up over the select menu's pop out. e.g.
radio-buttons. To demonstrate the problem, I copied demos/radio.html:72-94 into
demos/select.html just after line 74. This puts a radio button right
where the select menu pops up. When running the demo without the fix,
you can see the radio button over the select menu. With the z-index fix,
the menu appropriately pops up over the radio-button.

Fixes Issue 432

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[lukebergen]

I signed it!

[googlebot]

CLAs look good, thanks!

[codecov-io]

Codecov Report

Merging #460 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #460   +/-   ##
=======================================
  Coverage   99.82%   99.82%           
=======================================
  Files           5        5           
  Lines         568      568           
  Branches      151      153    +2     
=======================================
  Hits          567      567           
  Misses          1        1

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[traviskaufman]

Thanks for helping us out with this! LGTM. As a follow-up we've created #461 to get a more long-term strategy for this in place.