fix system date

fix for date bug.

```

in /home/kostya/crystal/src/compiler/crystal/version.cr:4: macro didn't expand to a valid program, it expanded to:

================================================================================
--------------------------------------------------------------------------------
  1. "\xxF\xzD. \xz0\xz2\xz3. 11 21:32:03 UTC 2014"
--------------------------------------------------------------------------------
Syntax error in expand macro:1: invalid hex escape

"\xxF\xzD. \xz0\xz2\xz3. 11 21:32:03 UTC 2014"
   ^

================================================================================

    build_date = {{ system("date -u").stringify }}

```