Switch license field in package.json to use SPDX identifier
Switch license field in package.json to use SPDX identifier

Reference: https://docs.npmjs.com/files/package.json#license

The NPM documentation (link above) suggests that the license under
which a project is released should be specified in the "license"
field within package.json. Without the presence of this field,
NPM will display an error whenever you add a package, reminding
you that your code (as far as NPM knows) is not licensed:

![image](https://cloud.githubusercontent.com/assets/442115/8896912/69b19930-33d8-11e5-9d50-b094987b8890.png)

Additionally, the NPM docs recommend the use of an SPDX identifier
(Software Package Data Exchange, https://spdx.org/, a standard for
communicating the license & copyright of a code package) for the
"license" field.

The license link in our "licenses" field is to the LGPL 2.1, so I
have switched package.json to use the SPDX equivalent, "LGPL-2.1"

OhJia/p5.js,bomoko/p5.js,mlarghydracept/p5.js,sakshamsaxena/p5.js,RomainRouphael/p5.js,bartuspan/p5.js,limzykenneth/p5.js,yupa884/p5.js,bomoko/p5.js,FreeSchoolHackers/p5.js,charneykaye/p5.js,mwcz/p5.js,futuremarc/p5.js,milrob/p5.js,yupa884/p5.js,johnpasquarello/p5.js,rolka/p5.js,Jared-Sprague/p5.js,mwcz/p5.js,PeterDaveHello/p5.js,111t8e/p5.js,limzykenneth/p5.js,picxenk/p5.js,plural/p5.js,processing/p5.js,joecridge/p5.js,futuremarc/p5.js,zenozeng/p5.js,processing/p5.js,antiboredom/p5.js,bartuspan/p5.js,jefarrell/p5.js,mlarghydracept/p5.js,FreeSchoolHackers/p5.js,indefinit/p5.js,dieface/p5.js,shiffman/p5.js,Jared-Sprague/p5.js,charneykaye/p5.js,beni55/p5.js,nucliweb/p5.js,eozinche/p5.js,maxkolyanov/p5.js,nucliweb/p5.js,poemusica/p5.js,eozinche/p5.js,dhowe/p5.js,beni55/p5.js,johnpasquarello/p5.js,joecridge/p5.js,kyoungchinseo/p5.js,karenpeng/p5.js,Mohammed90/p5.js,wufengyi/p5.js,RomainRouphael/p5.js,kennethdmiller3/p5.js,xyfeng/p5.js,dhowe/p5.js,therewasaguy/p5.js,plural/p5.js,laurenbenichou/p5.js,PeterDaveHello/p5.js,kadamwhite/p5.js,karenpeng/p5.js,davidnunez/p5.js,OhJia/p5.js,xyfeng/p5.js,shiffman/p5.js,black-kite/p5.js,dieface/p5.js,poemusica/p5.js,antiboredom/p5.js,maxkolyanov/p5.js,black-kite/p5.js,Mohammed90/p5.js,milrob/p5.js,kennethdmiller3/p5.js,rolka/p5.js,wufengyi/p5.js,jefarrell/p5.js,zenozeng/p5.js,kadamwhite/p5.js,laurenbenichou/p5.js,kyoungchinseo/p5.js,picxenk/p5.js,therewasaguy/p5.js,sakshamsaxena/p5.js,indefinit/p5.js,111t8e/p5.js,davidnunez/p5.js