(deps): upgrade to canvas@2, jsdom@15

- because canvas@1 is deprecated and is having install issues
  - see hhttps://travis-ci.org/agilgur5/react-signature-canvas/builds/650962136,
    https://travis-ci.org/agilgur5/react-signature-canvas/builds/658062587,
    etc

jest.config.js

@@ -1,4 +1,6 @@
 module.exports = {
+ // Jest 24 defaults to JSDOM 11, upgrade to 15 to support canvas@2
+ testEnvironment: 'jest-environment-jsdom-fifteen',
  setupFilesAfterEnv: [
  // configure enzyme w/ react adapter
  '<rootDir>/test/config/configure-enzyme.js',

package.json

@@ -63,10 +63,11 @@
  "babel-preset-es2015": "^6.14.0",
  "babel-preset-react": "^6.11.1",
  "babel-preset-stage-2": "^6.13.0",
- "canvas-prebuilt": "^1.6.11",
+ "canvas": "^2.6.1",
  "css-loader": "^0.24.0",
  "enzyme": "^3.10.0",
  "jest": "^24.8.0",
+ "jest-environment-jsdom-fifteen": "^1.0.2",
  "jest-without-globals": "^0.0.2",
  "react": "^17.0.2",
  "react-dom": "^17.0.2",