Use PyPy if available

It's slower to launch than CPython, but around 30% faster once running.

CPython:

```
$ bundle exec ruby bench.rb
Benchmarking....
Size: 15523 bytes
Iterations: 10
                                               user     system      total        real
pygments popen                               0.010000   0.010000   0.020000 (  0.555983)
pygments popen (process already started)     0.000000   0.000000   0.000000 (  0.241948)
pygments popen (process already started 2)   0.010000   0.000000   0.010000 (  0.232061)

$ bundle exec ruby bench.rb 20 25
Benchmarking....
Size: 388075 bytes
Iterations: 20
                                               user     system      total        real
pygments popen                               0.080000   0.040000   0.130000 ( 11.803712)
pygments popen (process already started)     0.090000   0.040000   0.130000 ( 11.600885)
pygments popen (process already started 2)   0.090000   0.040000   0.130000 ( 11.567272)
```

PyPy:

```
$ bundle exec ruby bench.rb
Benchmarking....
Size: 15523 bytes
Iterations: 10
                                               user     system      total        real
pygments popen                               0.010000   0.000000   0.020000 (  0.997430)
pygments popen (process already started)     0.010000   0.000000   0.010000 (  0.366487)
pygments popen (process already started 2)   0.000000   0.010000   0.010000 (  0.273575)

$ bundle exec ruby bench.rb 20 25
Benchmarking....
Size: 388075 bytes
Iterations: 20
                                               user     system      total        real
pygments popen                               0.080000   0.030000   0.110000 (  9.264323)
pygments popen (process already started)     0.080000   0.040000   0.120000 (  7.407072)
pygments popen (process already started 2)   0.080000   0.030000   0.110000 (  7.446369)
```

/cc @tmm1 @tnm