remove require of "ffi" on jruby.

a long long time ago "ffi" was automatically required by pry on jruby.
no one has a clear idea why.

it looks like without the require of ffi the tests still pass and pry
still boots:
```
$ jruby -S bundle exec pry
[1] pry(main)> require "ffi"
=> true
[2] pry(main)> $ Pry
....
```
```
$ jruby -v
jruby 1.7.9 (1.9.3p392)
```