Update gcc from 5.2 to 5.3

gcc 5.3 is out, so let's use the new one!

Signed-off-by: Christy Perez <christy@linux.vnet.ibm.com>

Dockerfile.gccgo

@@ -6,7 +6,7 @@
 # docker build -t docker -f Dockerfile.gccgo .
 #
 
-FROM gcc:5.2
+FROM gcc:5.3
 
 # Packaged dependencies
 RUN apt-get update && apt-get install -y \

Dockerfile.ppc64le

@@ -6,7 +6,7 @@
 # docker build -t docker -f Dockerfile.ppc64le .
 #
 
-FROM ppc64le/gcc:5.2
+FROM ppc64le/gcc:5.3
 
 # Packaged dependencies
 RUN apt-get update && apt-get install -y \