make the bootstrap script recognize automake 1.15

When using automake 1.15, the bootstrap script fails with a message saying that
the required version is 1.5 or newer. This patch fixes this.

bootstrap

@@ -36,7 +36,7 @@ aclocalflags="`sed -ne 's/^[ \t]*ACLOCAL_AMFLAGS[ \t]*=//p' Makefile.am`"
 
 # Check for automake
 amvers="no"
-for v in 14 13 12 11 10 9 8 7 6 5; do
+for v in 15 14 13 12 11 10 9 8 7 6 5; do
   if automake-1.${v} --version >/dev/null 2>&1; then
     amvers="-1.${v}"
     break