removed dependency on pbkdf2-compat

[killerstorm]

pbkdf2 functions are available via crypto, both the regular one and crypto-browserify.
Thus pbkdf2-compat is an unnecessary.

Actually, not just unnecessary but harmful, it fails in browserified form due to a circular dependency.

[weilu]

Thank you for the suggestion. Node's built-in pbkdf2 doesn't support custom digest methods until 0.11.11. BIP39 needs hmac-sha512 but node 0.10 only supports hmac-sha1, therefore the failed travis build. I'm happy to move on to the built-in cyrpto's pbkdf2 after node 0.12 is released.

testling passes on the current master. Can you elaborate on the browserify failure with steps to reproduce in a separate issue please?

[killerstorm]

Sure. It fails when pbkdf2-compat is loaded before crypto:

var pbkdf2_compat = require('pbkdf2-compat');
var crypto = require('crypto');
var bip39 = require('bip39');
alert(bip39.mnemonicToSeedHex('hello sadjk fljkasdfjkl sfdjak ', ''));

If you reverse the order of the first two lines, it works. Here's complete repro.

I still don't get why it happens that way in our code base, as we don't load pbkdf2-compat explicitly, and I was unable to find package which does it.