Updating version number to v3.1.0

Author: QuantumBadger

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "speedy2d"
-version = "3.0.0"
+version = "3.1.0"
 authors = ["QuantumBadger <speedy2d@redreader.org>"]
 edition = "2018"
 license = "Apache-2.0"

README.md

@@ -61,7 +61,7 @@ change `hello_world` to the name of the example source file).
 
 ```toml
 [dependencies]
-speedy2d = "2.1.0"
+speedy2d = "3.1.0"
 ```
 
 **Step 2:** Create a window: