Fix uniform percentiles

Percentiles in uniform metrics return the number of entries below that percentile, not the value itself.
I wonder if there is any reason for this, otherwise they should be computed as the histogram ones.

Current implementation:
```
12> exometer:new([c], uniform).                                          
ok
13> [exometer:update([c], X) || X  <- lists:seq(500, 510)].              
[ok,ok,ok,ok,ok,ok,ok,ok,ok,ok,ok]
14> exometer:get_values([c]).
[{[c],
  [{min,500},
   {max,510},
   {median,505},
   {mean,505.0},
   {50,6},
   {75,8},
   {90,10},
   {95,10},
   {99,11},
   {999,11}]}]
```

Proposed implementation:
```
17> exometer:get_values([c]).
[{[c],
  [{min,500},
   {max,510},
   {median,505},
   {mean,505.0},
   {50,505},
   {75,507},
   {90,509},
   {95,509},
   {99,510},
   {999,510}]}]
```