Add depsolver as a direct dependency

Adding depsolver as a direct dependency. Here's an audit of the usage of the depsolver library within chef_db to show that it uses non of the solving functions:

```
upsilon:chef_db stephen$ ack -a depsolver:
src/chef_db.erl
587:                                              OrgName :: binary()) -> [depsolver:dependency_set()] |

src/chef_sql.erl
624:-spec fetch_all_cookbook_version_dependencies(OrgId :: object_id()) -> {ok, [depsolver:dependency_set()]} |
680:                    {ok, Filtered} = depsolver:filter_packages(Packages, Constraints),
1531:-spec process_dependency_resultset(proplists:proplist()) -> [depsolver:dependency_set()].
1537:                                   WorkingDependencySet :: [depsolver:dependency_set()]) ->
1538:                                          FinalDependencySet :: [depsolver:dependency_set()].
1567:-spec row_to_dependency_set(Row :: proplists:proplist()) -> depsolver:dependency_set().
```