Add ESLint config file for tests (#41)

Instead of configuring rules per file using comments.
diff --git a/spec/.eslintrc.yml b/spec/.eslintrc.yml
new file mode 100644
index 0000000..77cb525
--- /dev/null
+++ b/spec/.eslintrc.yml
@@ -0,0 +1,4 @@
+env:
+    jasmine: true
+rules:
+    prefer-promise-reject-errors: off
diff --git a/spec/fetch-unit.spec.js b/spec/fetch-unit.spec.js
index a62c8e1..7bb9a04 100644
--- a/spec/fetch-unit.spec.js
+++ b/spec/fetch-unit.spec.js
@@ -15,9 +15,6 @@
     under the License.
 */
 
-/* eslint-env jasmine */
-/* eslint-disable prefer-promise-reject-errors */
-
 const rewire = require('rewire');
 
 describe('fetch', function () {
diff --git a/spec/fetch.spec.js b/spec/fetch.spec.js
index 0023eea..15b45a0 100644
--- a/spec/fetch.spec.js
+++ b/spec/fetch.spec.js
@@ -15,8 +15,6 @@
     under the License.
 */
 
-/* eslint-env jasmine */
-
 var fetch = require('..');
 var uninstall = fetch.uninstall;