Update nan package

[andiliu-gh]

This PR updates the nan package to fix compatibility issues when building with Electron 13.

With version 2.14.0 of nan package you encounter this error when trying to use electron-build-env:

In file included from ../src/neon.cc:2:
  In file included from ../node_modules/nan/nan.h:2884:
  ../node_modules/nan/nan_typedarray_contents.h:34:43: error: no member named 'GetContents' in 'v8::ArrayBuffer'
        data   = static_cast<char*>(buffer->GetContents().Data()) + byte_offset;
                                    ~~~~~~  ^
  In file included from ../src/neon.cc:8:
  ../src/neon_string.h:28:5: warning: ignoring return value of function declared with 'warn_unused_result' attribute [-Wunused-result]
      maybe.ToLocal(&result);
      ^~~~~~~~~~~~~ ~~~~~~~
  1 warning and 1 error generated.
  make: *** [Release/obj.target/neon/src/neon.o] Error 1

This is fixed when upgrading the nan package to version 2.15.0

This is related to this issue: #760 and PR: #769

[kjvalencik]

@andiliu-gh Thanks for this PR! Do you need this fix released urgently or could it wait until the next release?

[andiliu-gh]

Would be nice to have this released urgently as it blocks updating our app from Electron 11 -> 13!

[kjvalencik]

@andiliu-gh Are you able to be on the latest version of Neon or would you need a hotfix?

[andiliu-gh]

We are able to be on the latest version of Neon!

[kjvalencik]

@andiliu-gh This was published in v0.9.1. Thanks again for the fix!