hydra: Remove GCC 4.7 from the core set, and add 4.9 and 5.1.

* build-aux/hydra/gnu-system.scm (%core-packages): Remove GCC-4.7.  Add
  GCC-4.9 and GCC-5.1.

build-aux/hydra/gnu-system.scm

@@ -101,7 +101,7 @@ SYSTEM."
   ;; Note: Don't put the '-final' package variants because (1) that's
   ;; implicit, and (2) they cannot be cross-built (due to the explicit input
   ;; chain.)
-  (list gcc-4.8 gcc-4.7 glibc binutils
+  (list gcc-4.8 gcc-4.9 gcc-5.1 glibc binutils
         gmp mpfr mpc coreutils findutils diffutils patch sed grep
         gawk gnu-gettext hello guile-2.0 zlib gzip xz
         %bootstrap-binaries-tarball