Update to use log version 0.3.x

This was causing "multiple rlib candidates" errors when linking in a
project that also depended on a more recent log version.
diff --git a/Cargo.toml b/Cargo.toml
index dfd17e5..136d514 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -14,7 +14,7 @@
 keywords = ["random"]
 
 [dependencies]
-log = "0.2.1"
+log = "0.3.0"
 libc = "0.1.1"
 
 [dev-dependencies.rand_macros]