Made compass dependency more liberal to fix #26

I adjusted the compass dependency to fix [#26](https://github.com/Team-Sass/Sassy-math/issues/26)

```bash
Gem::LoadError on line ["2001"] of /home/yuvilio/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/site_ruby/2.0.0/rubygems/specification.rb: Unable to activate sassy-math-1.5, because compass-1.0.0.alpha.17 conflicts with compass (~> 0.11)
Run with --trace to see the full backtrace```

and rebuilt the gem in compass/ with 
```bash
$ gem build sassy-math.gemspec
WARNING:  licenses is empty.  Use a license abbreviation from:
  http://opensource.org/licenses/alphabetical
  Successfully built RubyGem
  Name: sassy-math
  Version: 1.5
  File: sassy-math-1.5.gem
$
```