Avoid sed with extended regexp

Seems like sed 4.1.5 on RHEL 5.3 doesn't support `-E`, and sed on OS X doesn't
support `-r` to activate the extended regexp mode. Best to avoid extended regexp
altogether for compatibility.

Fixes #495

bin/ruby-build

@@ -592,7 +592,9 @@ fix_rbx_gem_binstubs() {
     for file in "$gemdir"/*; do
       binstub="${bindir}/${file##*/}"
       rm -f "$binstub"
-      sed -E "s:^#\!.+:#\!${bindir}/ruby:" < "$file" > "$binstub"
+      { echo "#!${bindir}/ruby"
+        cat "$file"
+      } > "$binstub"
       chmod +x "$binstub"
     done
     rm -rf "$gemdir"

test/build.bats

@@ -424,6 +424,7 @@ OUT
   run cat "${INSTALL_ROOT}/bin/rake"
   assert_output <<OUT
 #!${INSTALL_ROOT}/bin/ruby
+#!rbx
 puts 'rake'
 OUT
 
@@ -431,6 +432,7 @@ OUT
   run cat "${INSTALL_ROOT}/bin/irb"
   assert_output <<OUT
 #!${INSTALL_ROOT}/bin/ruby
+#!rbx
 print '>>'
 OUT
 }