Use lower-case letters for github URLs.

This makes it a little more consistent; now, "cretonne" is never capitalized
in identifier, path, or URL contexts. It is capitalized in natural
language contexts when referring to the project.

lib/entity/Cargo.toml

@@ -5,7 +5,7 @@ version = "0.4.4"
 description = "Data structures using entity references as mapping keys"
 license = "Apache-2.0"
 documentation = "https://cretonne.readthedocs.io/"
-repository = "https://github.com/Cretonne/cretonne"
+repository = "https://github.com/cretonne/cretonne"
 readme = "README.md"
 keywords = ["entity", "set", "map"]
 
@@ -15,4 +15,4 @@ std = []
 
 [badges]
 maintenance = { status = "experimental" }
-travis-ci = { repository = "Cretonne/cretonne" }
+travis-ci = { repository = "cretonne/cretonne" }