Replace map.flatten(1) with flat_map

Using `flat_map` is equivalent `map.flatten(1)` but simpler and more than 4X faster.

###### Benchmark

```ruby
require 'benchmark/ips'

ARRAY = 100.times.map { (0..9).to_a }

def slow
  ARRAY.map { |x| x }.flatten(1)
end

def fast
  ARRAY.flat_map { |x| x }
end

Benchmark.ips do |x|
  x.report('slow') { slow }
  x.report('fast') { fast }
  x.compare!
end
```

###### Ruby 2.1.2

```
slow 12268.9 (±7.3%) i/s -  61182 in 5.013846s
fast 53921.4 (±7.1%) i/s - 269412 in 5.023821s

Comparison:
fast: 53921.4 i/s
slow: 12268.9 i/s - 4.39x slower
```

This is one instance of `map.flatten` (without the `1` argument) that I replaced with `flat_map` (in `lib/pry/wrapped_module.rb`), since it appears to always return one-dimensional arrays.