Fix debian dependency name mangling.

Tries to modify frozen string.

```
$ bundle exec fpm -s dir -t deb -n foo -v 1.0 -d 'Aafasf-asf_sf' --inputs foo
/home/cpk/src/ruby/fpm/lib/fpm/version.rb:2: warning: already initialized constant VERSION
Downcasing dependency 'Aafasf-asf_sf' because deb packages  don't work so good with uppercase names {:level=>:warn}
/home/cpk/src/ruby/fpm/lib/fpm/package/deb.rb:352:in `gsub!': string frozen (RuntimeError)
	from /home/cpk/src/ruby/fpm/lib/fpm/package/deb.rb:352:in `fix_dependency'
	from /home/cpk/src/ruby/fpm/lib/fpm/package/deb.rb:323:in `converted_from'
	from /home/cpk/src/ruby/fpm/lib/fpm/package/deb.rb:322:in `collect'
	from /home/cpk/src/ruby/fpm/lib/fpm/package/deb.rb:322:in `converted_from'
	from /home/cpk/src/ruby/fpm/lib/fpm/package.rb:208:in `convert'
	from /home/cpk/src/ruby/fpm/lib/fpm/command.rb:396:in `execute'
	from /home/cpk/.rvm/gems/ruby-1.8.7-p249/gems/clamp-0.3.1/lib/clamp/command.rb:64:in `run'
	from /home/cpk/.rvm/gems/ruby-1.8.7-p249/gems/clamp-0.3.1/lib/clamp/command.rb:126:in `run'
	from /home/cpk/src/ruby/fpm/bin/fpm:8
	from /home/cpk/.rvm/gems/ruby-1.8.7-p249/bin/fpm:19:in `load'
	from /home/cpk/.rvm/gems/ruby-1.8.7-p249/bin/fpm:19
	from /home/cpk/.rvm/gems/ruby-1.8.7-p249/bin/ruby_noexec_wrapper:14

$ bundle exec fpm -s dir -t deb -n foo -v 1.0 -d 'afasf-asf_sf' --inputs foo
/home/cpk/src/ruby/fpm/lib/fpm/version.rb:2: warning: already initialized constant VERSION
Replacing underscores with dashes in 'afasf-asf_sf' because debs don't like underscores {:level=>:warn}
/home/cpk/src/ruby/fpm/lib/fpm/package/deb.rb:358:in `gsub!': can't modify frozen string (TypeError)
	from /home/cpk/src/ruby/fpm/lib/fpm/package/deb.rb:358:in `fix_dependency'
	from /home/cpk/src/ruby/fpm/lib/fpm/package/deb.rb:323:in `converted_from'
	from /home/cpk/src/ruby/fpm/lib/fpm/package/deb.rb:322:in `collect'
	from /home/cpk/src/ruby/fpm/lib/fpm/package/deb.rb:322:in `converted_from'
	from /home/cpk/src/ruby/fpm/lib/fpm/package.rb:208:in `convert'
	from /home/cpk/src/ruby/fpm/lib/fpm/command.rb:396:in `execute'
	from /home/cpk/.rvm/gems/ruby-1.8.7-p249/gems/clamp-0.3.1/lib/clamp/command.rb:64:in `run'
	from /home/cpk/.rvm/gems/ruby-1.8.7-p249/gems/clamp-0.3.1/lib/clamp/command.rb:126:in `run'
	from /home/cpk/src/ruby/fpm/bin/fpm:8
	from /home/cpk/.rvm/gems/ruby-1.8.7-p249/bin/fpm:19:in `load'
	from /home/cpk/.rvm/gems/ruby-1.8.7-p249/bin/fpm:19
	from /home/cpk/.rvm/gems/ruby-1.8.7-p249/bin/ruby_noexec_wrapper:14
```