Drop unicode support

[Angelmmiguel]

Until now, all versions of this gem were released with two possible configurations:

• Ligatures
• Unicode

The main reason was that at the time I created it, ligatures was not completely adopted yet. However, now the support has improved a lot and browsers are already enabling it by default (See Ligatures support in "can I use?"].

Adding unicode support requires to create a separate CSS file based on the list of codepoints from the Material Icons project. This CSS file is much heavier and it should be used by default.

With a 96% of support based in the can I use? data, it's time to drop this feature for future versions of material icons.