Need to install freetype headers on Ubuntu

[caspark]

PR to update the Ubuntu instructions to account for #40 : I got a message about missing freetype headers when I tried to compile on my lubuntu development vm, and installing libfreetype6-dev fixed it.

As mentioned in that issue, maybe the correct fix here is to figure out what introduced the hard dependency on freetype-rs (which is presumably what depends on the freetype headers) and remove that instead?

Since PR #46 included instructions for freetype on windows I figured I'd raise this anyway.

[Potpourri]

opengl_graphics require freetype to realize glyph cache, this uses now only in conrod
Maybe worth move glyph cache to self repo @bvssvni ?

[bvssvni]

@Potpourri It would still only work with the opengl_graphics back-end, because CharacterCache::Texture points to the texture type. If we had a trait for creating textures in piston-texture, then we could move GlyphCache to its own library and auto implement it. Will open up an issue about it.

[bvssvni]

@caspark Thanks!