Make getting time work in older bash.

Using printf to get the time is only avalible in fairly new bash (bash 4.1.2
doesn't seem to have it).  It gives the following error:
  % printf -v now "%(%s)T" -1
  -bash: printf: `(': invalid format character

When this is not avaliable, fall back to slower date command.
Merge pull request #19 from mikey/old-bash-fix

Make getting time work in older bash. (mikey)