Fix race condition in cover.erl

Erlang R15B02 and R16B has a race condition in cover.erl, when a project with several applications in the apps directory is launched throught rebar with eunit, often crash with this message:

```erlang
ERROR: eunit failed while processing /home/manuel/Proyectos/myproject/apps/app2: {'EXIT',
    {{badmatch,
         {error,
             {badarg,
                 [{ets,new,
                      [cover_internal_data_table,
                       [set,public,named_table,{write_concurrency,true}]],
                      []},
                  {cover,init_main,1,[{file,"cover.erl"},{line,565}]}]}}},
     [{rebar_eunit,cover_init,2,[{file,"src/rebar_eunit.erl"},{line,445}]},
      {rebar_eunit,run_eunit,3,[{file,"src/rebar_eunit.erl"},{line,127}]},
      {rebar_core,run_modules,4,[{file,"src/rebar_core.erl"},{line,405}]},
      {rebar_core,execute,5,[{file,"src/rebar_core.erl"},{line,334}]},
      {rebar_core,process_dir1,6,[{file,"src/rebar_core.erl"},{line,197}]},
      {rebar_core,process_each,5,[{file,"src/rebar_core.erl"},{line,268}]},
      {rebar_core,process_dir1,6,[{file,"src/rebar_core.erl"},{line,173}]},
      {rebar_core,process_commands,2,
          [{file,"src/rebar_core.erl"},{line,61}]}]}}
```
Adding cover.erl to the "src" directory fix the issue.