Add script to create gzipped file in development & report gzip size on weigh in

[kevinzwhuang]

Description

Currently, react-apollo uses gzip-size to get the gzip size without writing a .gz file to disk. travis-weigh-in requires an actual file to work with, making it incompatible with gzip-size.

To solve this, this PR replaces gzip-size with a simple script that saves the compressed gzip and uses that file for file size calculation for both npm run filesize and travis-weigh-in scripts.

The compression strategy utilized by /scripts/gzip.js is identical to what's used in gzip-size, with both using zlib set to level 9. The main change is that gzip.js creates a .gz file.

Also, the added .gz file in /dist might be helpful for people wanting to use gzipped versions of react-apollo in development.

Summary of changes:

• Add a simple gzip.js script to /scripts to create compressed gzips (used by compress:browser)
• Add compress:browser script to package.json to handle creation of compressed index.js.gz for consumption by the filesize and travis-weigh-in scripts
• Append npm run compress:browser to the end of compile:browser
• Remove gzip-size as a devDependency

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This PR addresses #574

[helfer]

This looks great. Thanks @kevinzwhuang !