Change hasBlock to has-block to avoid deprecation

[wagenet]

I'm getting this error in one of my apps:

DEPRECATION: `hasBlock` is deprecated. Use `has-block` instead. ('ember-cli-flash/templates/components/flash-message.hbs' @ L9:C8)  [deprecation id: has-block-and-has-block-params] See https://deprecations.emberjs.com/v3.x#toc_has-block-and-has-block-params for more details.
        at logDeprecationStackTrace (/Users/peterwagenet/Development/Tilde/events-app/node_modules/ember-source/dist/ember-template-compiler.js:1984:21)
        at HANDLERS.<computed> (/Users/peterwagenet/Development/Tilde/events-app/node_modules/ember-source/dist/ember-template-compiler.js:2117:9)
        at raiseOnDeprecation (/Users/peterwagenet/Development/Tilde/events-app/node_modules/ember-source/dist/ember-template-compiler.js:2011:9)
        at HANDLERS.<computed> (/Users/peterwagenet/Development/Tilde/events-app/node_modules/ember-source/dist/ember-template-compiler.js:2117:9)
        at invoke (/Users/peterwagenet/Development/Tilde/events-app/node_modules/ember-source/dist/ember-template-compiler.js:2129:9)
        at deprecate (/Users/peterwagenet/Development/Tilde/events-app/node_modules/ember-source/dist/ember-template-compiler.js:2085:28)
        at emitDeprecationMessage (/Users/peterwagenet/Development/Tilde/events-app/node_modules/ember-source/dist/ember-template-compiler.js:19966:49)
        at PathExpression (/Users/peterwagenet/Development/Tilde/events-app/node_modules/ember-source/dist/ember-template-compiler.js:19982:13)
        at visitNode (/Users/peterwagenet/Development/Tilde/events-app/node_modules/ember-source/dist/ember-template-compiler.js:11855:16)

I'm hoping that this will help work around it.