[fennec] upgrade to GCC 6.1 or newer

[jankeromnes]

The Fennec build is currently broken on Circle CI:

 0:02.06 checking the target C compiler version... 6.0.0
 0:02.24 checking the target C compiler works... yes
 0:02.24 checking for the target C++ compiler... /usr/bin/clang++-6.0
 0:02.32 checking whether the target C++ compiler can be used... yes
 0:02.32 checking the target C++ compiler version... 6.0.0
 0:02.39 checking the target C++ compiler works... yes
 0:02.39 checking for the host C compiler... /usr/bin/gcc
 0:02.42 checking whether the host C compiler can be used... no
 0:02.43 ERROR: Only GCC 6.1 or newer is supported (found version 5.4.0).
 0:02.45 *** Fix above errors and then restart with\
 0:02.45                "/usr/bin/make -f client.mk build"
 0:02.45 client.mk:145: recipe for target 'configure' failed
 0:02.45 make: *** [configure] Error 1
The command '/bin/sh -c ./mach build  && ./mach package' returned a non-zero code: 2
Exited with code 2