Fix invalid block warning text overflow in IE11

[aduth]

Fixes #2073
See: https://stackoverflow.com/a/35113633

This pull request seeks to resolve a styling issue which occurs in the invalid block warning, affecting IE11 only.

Before	After

Testing instructions:

Repeat steps to reproduce from #2073, verifying that an invalid block warning displays correctly in IE11.

[codecov]

Codecov Report

Merging #2074 into master will decrease coverage by 0.57%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2074      +/-   ##
==========================================
- Coverage   20.25%   19.68%   -0.58%     
==========================================
  Files         131      131              
  Lines        4207     4441     +234     
  Branches      716      800      +84     
==========================================
+ Hits          852      874      +22     
- Misses       2827     2959     +132     
- Partials      528      608      +80

Impacted Files	Coverage Δ
blocks/editable/index.js	0.29% <0%> (-0.18%)	⬇️
editor/modes/visual-editor/index.js	0% <0%> (ø)	⬆️
editor/index.js	0% <0%> (ø)	⬆️
editor/modes/visual-editor/block.js	0% <0%> (ø)	⬆️
editor/utils/undoable-reducer.js	100% <0%> (+3.22%)	⬆️
editor/actions.js	41.66% <0%> (+4.62%)	⬆️

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[nylen]

I would have expected codecov to ignore this PR entirely. As of #2092 it will not leave comments on PRs that do not change coverage, and at https://github.com/codecov/support/issues/427 I've asked for some support regarding why this PR was reported as a coverage change.