Fix quoting in pre_hooks to get it copmiling when folder name has spaces.

rebar.config

@@ -26,11 +26,11 @@
  ]}.
 
 {pre_hooks,
- [{compile, "make -C $REBAR_DEPS_DIR/merl all -W test"},
-  {"freebsd", compile, "gmake -C $REBAR_DEPS_DIR/merl all"},
+ [{compile, "make -C \"$REBAR_DEPS_DIR/merl\" all -W test"},
+  {"freebsd", compile, "gmake -C \"$REBAR_DEPS_DIR/merl\" all"},
   {eunit,
    "erlc -I include/erlydtl_preparser.hrl -o test"
    " test/erlydtl_extension_testparser.yrl"},
-  {eunit, "make -C $REBAR_DEPS_DIR/merl test"},
-  {"freebsd", eunit, "gmake -C $REBAR_DEPS_DIR/merl test"}
+  {eunit, "make -C \"$REBAR_DEPS_DIR/merl\" test"},
+  {"freebsd", eunit, "gmake -C \"$REBAR_DEPS_DIR/merl\" test"}
  ]}.