Resolve default package definitions if extact version is unavailable.
Old behavior can be reverted by setting up a dependency_resolvers_conf.xml with the following contents

```
<dependency_resolvers>
  <tool_shed_packages />
  <galaxy_packages />
</dependency_resolvers>
```

The new behavior corresponds to the following dependency_resolvers_conf.xml contents:

```
<dependency_resolvers>
  <tool_shed_packages />
  <galaxy_packages />
  <galaxy_packages versionless="true" />
</dependency_resolvers>
```

Still think galaxy_packages should come before tool_shed_packages in resolution order so that deployers can fix broken tool shed installs with manual installs or deploy optimized versions of packages without having to mess with a dependency_resolvers_conf.xml file.
Merged in jmchilton/galaxy-central-fork-1 (pull request #272)

Resolve default package definitions if exact version is unavailable.