Bump minimum Node version to 7.6, which has non-harmony support for async/await; make async tests always enabled, not just for harmony mode

Cakefile

@@ -403,11 +403,6 @@ runTests = (CoffeeScript) ->
   # Run every test in the `test` folder, recording failures.
   files = fs.readdirSync 'test'
 
-  # Ignore async test file if async/await is not available
-  asyncSupported = parseInt(process.versions.node.split('.')[0]) >= 7 and
-    ('--harmony' in process.execArgv or '--harmony-async-await' in process.execArgv)
-  files.splice files.indexOf('async.coffee'), 1 unless asyncSupported
-
   for file in files when helpers.isCoffee file
     literate = helpers.isLiterate file
     currentFile = filename = path.join 'test', file

package.json

@@ -11,7 +11,7 @@
   "version": "2.0.0-alpha1",
   "license": "MIT",
   "engines": {
-    "node": ">=7.2.1"
+    "node": ">=7.6.0"
   },
   "directories": {
     "lib": "./lib/coffeescript"