Use `patch --force` to ignore missing files when patching

From patch(1):

    -f  or  --force
       Assume that the user knows exactly what he or she is doing, and do  not
       ask any questions.  Skip patches whose headers do not say which file is
       to be patched; patch files even though they have the wrong version  for
       the Prereq: line in the patch; and assume that patches are not reversed
       even if they look like they are.  This option does not suppress commen-
       tary; use -s for that.

Fixes #555

bin/ruby-build

@@ -848,7 +848,7 @@ isolated_gem_install() {
 apply_ruby_patch() {
   case "$1" in
   ruby-* | jruby-* | rubinius-* )
-    patch -p0 -i "${2:--}"
+    patch -p0 --force -i "${2:--}"
     ;;
   esac
 }

test/build.bats

@@ -97,7 +97,7 @@ OUT
 yaml-0.1.6: --prefix=$INSTALL_ROOT
 make -j 2
 make install
-patch -p0 -i -
+patch -p0 --force -i -
 ruby-2.0.0: --prefix=$INSTALL_ROOT
 make -j 2
 make install