Remove black line from progress bar in IE

[rafeememon]

This is a bugfix for inconsistent progress bar styling in IE.

Proposed solution

This PR removes the border from the ::-ms-fill pseudo-element of the progress bar. The problem and solution are discussed in this SO post: https://stackoverflow.com/questions/21780059/why-are-my-progress-bars-displaying-incorrectly-in-ie

Fixes #1340.

Tradeoffs

This adds one additional declaration to the build, but improves support for IE.

Testing Done

I built my branch of Bulma locally, added it to my project, and confirmed that the progress bar looks as intended.

Before fix:

After fix:

[rafeememon]

Thanks!