Update phf to 0.12

Depends on servo/string-cache#295

Author: xu-cheng

Cargo.toml

@@ -32,10 +32,10 @@ proc-macro2 = "1"
 log = "0.4"
 mac = "0.1"
 tendril = "0.4"
-string_cache = "0.8.8"
-string_cache_codegen = "0.5.4"
-phf = "0.11"
-phf_codegen = "0.11"
+string_cache = "0.9.0"
+string_cache_codegen = "0.6.0"
+phf = "0.12"
+phf_codegen = "0.12"
 
 # Dev dependencies
 criterion = "0.6"