Remove GCC 4.x and 5.x from the CI configuration.

After commit a022cc0 removed the `-std` flag from Makefile (and re2.pc),
these compiler versions are empirically too old.

Change-Id: I51321b9ce62324b32d7b70a2466af304389a53fd
Reviewed-on: https://code-review.googlesource.com/c/re2/+/59590
Reviewed-by: Emil Mikulic <easy@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>
1 file changed