bump rust-sdl2 to 0.28

sdl2-sys is kept at version 0.27 and this is intended,
there were no changes to sdl2-sys since 0.27, hence updating
is useless for now. If sdl2-sys is updated in sdl2 0.29,
sdl2-sys will be updated to 0.29 as well, not 0.28

Cargo.toml

@@ -4,7 +4,7 @@ name    = "sdl2"
 description = "SDL2 bindings for Rust"
 repository = "https://github.com/AngryLawyer/rust-sdl2"
 documentation = "http://angrylawyer.github.io/rust-sdl2/sdl2/"
-version = "0.27.3"
+version = "0.28.0"
 license = "MIT"
 authors = [ "Tony Aldridge <tony@angry-lawyer.com>" ]
 keywords = ["SDL", "windowing", "graphics"]

README.md

@@ -148,7 +148,7 @@ download through Crates.io:
 
 ```toml
     [dependencies]
-    sdl2 = "0.27"
+    sdl2 = "0.28"
 ```
 
 Alternatively, pull it from GitHub to obtain the latest version from master
@@ -169,7 +169,7 @@ adding this instead:
 
 ```toml
     [dependencies.sdl2]
-    version = "0.27"
+    version = "0.28"
     default-features = false
     features = ["ttf","image","gfx","mixer"]
 ```