Exclude .nycrc.yml from npm package (#1069)

This patch excludes the following files from npm package:
- `.nycrc.yml`
- `.gitattributes`

Fix: #1068

Author: minggangw

.npmignore

@@ -1,5 +1,7 @@
 .coveralls.yml
 .clang-format
+.gitattributes
+.nycrc.yml
 .istanbul.yml
 .gitignore
 .prettierrc.yml