Fix: ensure "version" always has a valid value in parseOptions (#109)

The rule no-unsupported-features would sometimes fail with a TypeError:
```
Error while loading rule 'node/no-unsupported-features': Cannot read property 'split' of null
TypeError: Error while loading rule 'node/no-unsupported-features': Cannot read property 'split' of null
    at new Range (.../node_modules/semver/semver.js:780:20)
    at Function.intersects (.../node_modules/semver/semver.js:1305:8)
    at Object.freeze.features.Object.freeze.OPTIONS.reduce (.../node_modules/eslint-plugin-node/lib/rules/no-unsupported-features.js:191:44)
    at Array.reduce (<anonymous>)
    at parseOptions (.../node_modules/eslint-plugin-node/lib/rules/no-unsupported-features.js:175:41)
    at Object.create (.../node_modules/eslint-plugin-node/lib/rules/no-unsupported-features.js:281:25)
```

This happens, when neither "engines" are set in the "package.json",
nor "version" is set in the rule configuration but the rule still has
configuration options, because version will then be set to
`options.version`, even if that is `undefined`.

This commit ensures that `version` always has a valid value before
trying to construct a semver range from it.

lib/rules/no-unsupported-features.js

@@ -104,7 +104,7 @@ function getDefaultVersion(filename) {
     const info = getPackageJson(filename)
     const nodeVersion = info && info.engines && info.engines.node
 
-    return nodeVersion ? semver.validRange(nodeVersion) : null
+    return semver.validRange(nodeVersion) || DEFAULT_VERSION
 }
 
 /**
@@ -147,7 +147,7 @@ function isIgnored(key, ignores) {
  * @returns {object} Parsed value.
  */
 function parseOptions(options, defaultVersion) {
-    let version = defaultVersion ? null : DEFAULT_VERSION
+    let version = null
     let range = null
     let ignores = []