Added CanvasPattern global flag for jshint
Added CanvasPattern global flag for jshint

The core.helpers file was failing linting checks as the global CanvasPattern was not defined. Added the `/* global CanvasGradient */` statement to make linting pass. Updates chartjs/Chart.js#1323

Magnetic/Chart.js,nicktiberi/Chart.js,supernovel/Chart.js,rvaidya/Chart.js,nnnick/Chart.js,2Ring/Chart.js,touletan/Chart.js,simonbrunel/Chart.js,touletan/Chart.js,jeffesp/Chart.js,2Ring/Chart.js,ritesrnjn/charts,pombredanne/Chart.js,ldwformat/Chart.js,panzarino/Chart.js,potatopeelings/Chart.js,ldwformat/Chart.js,el-ee/Chart.js,nicktiberi/Chart.js,chartjs/Chart.js,simonbrunel/Chart.js,amir-hadzic/Chart.js,ashiguruma/Chart.js,chartjs/Chart.js,chartjs/Chart.js,amir-hadzic/Chart.js,touletan/Chart.js,potatopeelings/Chart.js,salzhrani/Chart.js,Magnetic/Chart.js,chartjs/Chart.js,ritesrnjn/charts,jtcorbett/Chart.js,salzhrani/Chart.js,el-ee/Chart.js,supernovel/Chart.js,bokysan/Chart.js,jtcorbett/Chart.js,thebenedict/Chart.js,avlcodemonkey/Chart.js,rvaidya/Chart.js,touletan/Chart.js,avlcodemonkey/Chart.js,bokysan/Chart.js,panzarino/Chart.js