Fix double bitcore-lib loading issue.
Fix double bitcore-lib loading issue.

Browserify has an option to exclude a library from the browser package
so that it can be loaded only once, previously we used `--external bitcore`
however now we need to use `--external bitcore-lib` as the package has
been renamed.

bitjson/bitcore,martindale/bitcore,bitpay/bitcore,bitpay/bitcore,bitpay/bitcore,bitpay/bitcore,martindale/bitcore,martindale/bitcore,bitjson/bitcore,martindale/bitcore,bitjson/bitcore,bitjson/bitcore