bump gcc for bootstrap

On Windows, the gcc crate would send /Wall to msvc, which would cause
builds to get flooded with warnings, exploding compile times from one
hour to more than 72! The gcc crate version 0.3.54 changes this behavior
to send /W4 instead, which greatly cuts down on cl.exe flooding the
command prompt window with warnings.

Author: QuietMisdreavus

src/Cargo.lock

@@ -34,7 +34,7 @@ cmake = "0.1.23"
 filetime = "0.1"
 num_cpus = "1.0"
 getopts = "0.2"
-gcc = "0.3.50"
+gcc = "0.3.54"
 libc = "0.2"
 serde = "1.0.8"
 serde_derive = "1.0.8"