fix(merge-method): aligned to the final version of defining merge-method

rebase is not yet supported. details of the api release:
https://developer.github.com/changes/2017-04-07-end-merge-methods-api-preview/

for #19

src/github/actions.js

@@ -52,7 +52,7 @@ export default function (githubCredentials) {
       sha,
       commit_title: `greenkeeper-keeper(pr: ${prNumber}): :white_check_mark:`,
       commit_message: `greenkeeper-keeper(pr: ${prNumber}): :white_check_mark:`,
-      squash
+      merge_method: squash ? 'squash' : 'merge'
     }).then(result => {
       log(['info', 'PR', 'integrated'], url);
       return result;

test/integration/features/step_definitions/github-api-steps.js

@@ -87,7 +87,7 @@ defineSupportCode(({Before, After, Given, setWorldConstructor}) => {
           sha: this.sha,
           commit_title: `greenkeeper-keeper(pr: ${this.prNumber}): :white_check_mark:`,
           commit_message: `greenkeeper-keeper(pr: ${this.prNumber}): :white_check_mark:`,
-          squash: this.squash
+          merge_method: this.squash ? 'squash' : 'merge'
         })
         .reply(OK, uri => {
           this.mergeUri = uri;

test/unit/github/actions-test.js

@@ -108,12 +108,24 @@ suite('github actions', () => {
 
   suite('accept PR', () => {
     test('that the referenced PR gets accepted', () => {
-      const squash = any.boolean();
+      const squash = false;
       put.withArgs(`${url}/merge`, {
         sha,
         commit_title: `greenkeeper-keeper(pr: ${prNumber}): :white_check_mark:`,
         commit_message: `greenkeeper-keeper(pr: ${prNumber}): :white_check_mark:`,
-        squash
+        merge_method: 'merge'
+      }).resolves(response);
+
+      return assert.becomes(actions.acceptPR(url, sha, prNumber, squash, log), response);
+    });
+
+    test('that the referenced PR gets squashed when configured to do so', () => {
+      const squash = true;
+      put.withArgs(`${url}/merge`, {
+        sha,
+        commit_title: `greenkeeper-keeper(pr: ${prNumber}): :white_check_mark:`,
+        commit_message: `greenkeeper-keeper(pr: ${prNumber}): :white_check_mark:`,
+        merge_method: 'squash'
       }).resolves(response);
 
       return assert.becomes(actions.acceptPR(url, sha, prNumber, squash, log), response);