Fix for Provides section with underscores

This pull request fixes a problem with calculating the Provides section of the debian control file when the source package contains an underscore.

For example:

```
$ fpm -s gem -t deb daemon_controller
```
generates a control file with the following:

```
$ dpkg -I rubygem-daemon-controller_1.1.7_all.deb | grep Provides
 Provides: rubygem-daemon_controller
```

instead of the expected "rubygem-daemon-controller"