fix: license should be a valid SPDX license expression

when `npm install` on the repo or a fork, npm complains that license
field in package.json is not valid.

See https://docs.npmjs.com/files/package.json#license
for valid `license` expressions

Same applies for bower.json:
https://github.com/bower/spec/blob/master/json.md#license

More generally, both must be a string matching a vlid SPDX identifier:
https://spdx.org/licenses/

bower.json

@@ -22,10 +22,7 @@
     "url": "git://github.com/goldfire/howler.js.git"
   },
   "main": "dist/howler.js",
-  "license": {
-    "type": "MIT",
-    "url": "https://raw.githubusercontent.com/goldfire/howler.js/master/LICENSE.md"
-  },
+  "license": "MIT",
   "moduleType": [
     "amd",
     "globals",

package.json

@@ -29,10 +29,7 @@
   },
   "main": "dist/howler.js",
   "version": "2.0.3",
-  "license": {
-    "type": "MIT",
-    "url": "https://raw.githubusercontent.com/goldfire/howler.js/master/LICENSE.md"
-  },
+  "license": "MIT",
   "files": [
     "src",
     "dist/howler.js",