Reduce dependency size

[intelliot]

We can cut the size of the npm tarball by about 78% by excluding the other files in build/ (including only ripple-latest-min.js, which I think is the one that people should use most of the time).

• Remove .npmignore since it is overridden by package.json 'files'
• Include build/ripple-latest-min.js, not everything in build/

Before:

% npm publish --dry-run
[...]
npm notice === Tarball Details === 
npm notice name:          ripple-lib                              
npm notice version:       1.4.0-b2                                
npm notice package size:  1.9 MB                                  
npm notice unpacked size: 7.8 MB                                  
npm notice shasum:        5a28c5b629bdfa7299381d9d639673d0407f3233
npm notice integrity:     sha512-90mGTucpK5CcR[...]onwZt9ReLv7Yg==
npm notice total files:   570

After:

% npm publish --dry-run
[...]
npm notice === Tarball Details === 
npm notice name:          ripple-lib                              
npm notice version:       1.4.0-b2                                
npm notice package size:  419.5 kB                                
npm notice unpacked size: 1.6 MB                                  
npm notice shasum:        7ca4fc567916666447363a9be47f5305a727ba77
npm notice integrity:     sha512-VuyZ7b1T1gOOC[...]lmX8O4/96Dstw==
npm notice total files:   566