Mark the 0.6.0-pre.0 release

This includes most functional changes required for 0.6 but not documentaiton.
diff --git a/Cargo.toml b/Cargo.toml
index 616a3cd..66df440 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "rand"
-version = "0.5.5" # NB: When modifying, also modify html_root_url in lib.rs
+version = "0.6.0-pre.0" # NB: When modifying, also modify html_root_url in lib.rs
 authors = ["The Rand Project Developers", "The Rust Project Developers"]
 license = "MIT/Apache-2.0"
 readme = "README.md"
diff --git a/src/lib.rs b/src/lib.rs
index 19dbd63..122724e 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -218,7 +218,7 @@
 
 #![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk.png",
        html_favicon_url = "https://www.rust-lang.org/favicon.ico",
-       html_root_url = "https://docs.rs/rand/0.5.5")]
+       html_root_url = "https://docs.rs/rand/0.6.0-pre.0")]
 
 #![deny(missing_docs)]
 #![deny(missing_debug_implementations)]